National Academies Press: OpenBook

Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267 (2002)

Chapter: 2. Past Evaluations of Changes in Truck Size and Weight Regulations

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
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Chapter 2
Past Evaluations of Changes in Truck Size and Weight Regulations

In this chapter, a review of past evaluations of changes in truck size and weight regulations is presented. This review reveals that the estimates of some impacts of incremental regulatory changes provided in the studies of DOT, TRB, and others have been well founded and can help in making informed choices among alternatives. However, certain important impacts are poorly understood or have not been assessed with the most appropriate methods in these studies. For these impacts, proposals are made for obtaining the information needed for better assessments in the future. The present study has not produced new estimates of the impacts of changes in the regulations. The available models have been fully exercised in past studies, and resources were not available to the committee for the development of new methods.

The review in this chapter also points to two shortcomings common in past studies that are more fundamental than inadequacies of engineering and economic models and data. First, analyses have not started with clear definitions of the objectives of the regulations. Second, the analysis of changes in truck characteristics has not been integrated with the ongoing process of management and regulation of the highway system. As a consequence of these shortcomings, past studies, even when they have produced reasonable estimates of the consequences of changes in truck dimensions, often have not been successful in the design of improved policies or promotion of reform.

In the first section below, the evaluation framework that has become standard in past U.S. studies of truck size and weight regulation is described, and the two shortcomings identified above are examined. An overview of the evaluations of past studies is presented in the second section. In the third section, a detailed review of the estimation methods used for these evaluations is presented, including the most important needed improvements and the results obtained.

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

PROBLEMS IN PREDICTING IMPACTS OF CHANGES IN REGULATIONS

The DOT and TRB truck size and weight studies of the past 20 years employed a common five-step analysis method:

  1. One or more alternative sets of size and weight limits are specified.

  2. Projections are made of the changes in truck traffic volume and in the distribution of dimensions of vehicles in use that would result from introducing the alternative limits.

  3. The magnitudes of the changes in certain costs arising from the projected traffic changes—including pavement and bridge construction and maintenance, numbers of highway accidents, highway user delay, freight transportation costs, and air and noise pollution—are predicted.

  4. Certain practical issues, such as enforcement and administrative feasibility, fiscal impact on state highway programs, and effects on railroads, are given at least qualitative consideration.

  5. Recommendations are made for changes in limits on the basis of predicted economic benefits and recognized practical constraints.

Within this benefit–cost framework, the DOT (2000) Comprehensive Truck Size and Weight Study, as well as most of its predecessors, is constrained by strong assumptions about the scope of policy changes to be considered. Specifically, the DOT study assumes:

  • Constant highway user tax rates—Changes in size and weight limits would not be accompanied by any change in the user tax structure.

  • Constant motor vehicle technology—New trucks would be built with off-the-shelf components.

  • Constant highway design and construction practices—Highway agencies would continue to follow established practices in design of pavements, bridges, and road geometry.

  • Traditional regulatory structure—The form of size and weight rules and the dimensions regulated would remain unchanged; only the numerical values of limits would change.

The specific evaluation criteria that have been applied (see Box 2-1), together with the set of regulatory options (such as those presented earlier in Box 1-1), define a matrix with criteria as rows and options

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
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Box2-1 Evaluation Criteria

Criteria Considered in Past DOT or TRB Studies

  • Highway agency pavement costs: change in costs of maintenance and construction of pavement caused by change in vehicle dimensions

  • Highway agency bridge costs: cost of bridge replacements required (or avoided) by change in dimensions; change in future bridge construction and maintenance costs

  • Highway agency geometric improvement costs: cost of reconstruction to accommodate new vehicle dimensions; cost of changes in design of future highway projects necessitated by change in dimensions

  • Accident costs: change in costs of accidents not borne by carriers or shippers

  • Delay at construction: change in highway user delay caused by change in the amount of highway construction

  • Delay from effect on traffic operations: change in delay caused by change in number and performance of trucks

  • Air pollution: cost of change in emissions caused by change in traffic volume, vehicle performance, and highway construction

  • Noise: cost of change in noise emissions

  • Energy consumption: external costs (if any) of change in petroleum consumption, other than pollution costs

  • Railroad profitability: change in welfare of railroad stock-holders and employees (a distribution effect rather than a cost)

  • Shipper costs: net shipper benefits

Other Criteria

  • Other costs

    • Costs to road users other than accident and delay costs

    • Potential for unpredicted consequences

  • Summary measures of merit

    • Benefit/cost ratio or net present value

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×
  • Equity

    • Regional and local distribution of costs and benefits

    • Distribution of costs and benefits among shippers, carriers, public

  • Feasibility

    • Enactment feasibility

    • Implementation and enforcement feasibility

    • Provision of incentives for efficient use of highways

    • Appropriateness of federal involvement

as columns. The standard evaluation framework consists of filling in the cells of this matrix with quantitative estimates of the magnitudes of each category of impact (the criteria) for each policy option.

The past DOT and TRB studies applying this method have reached a similar conclusion: that incremental increases in allowable truck size would produce net benefits. Predicted increases in infrastructure costs (mainly for upgrading bridges) generally are smaller than predicted freight cost savings; and predicted safety, traffic, and pollution effects are often positive because increasing truck capacity is predicted to reduce total truck-miles of travel. A partial exception to this pattern of results, the DOT 2000 study estimates that the high cost of traffic delay caused by bridge construction would cancel freight productivity benefits for some changes in limits that otherwise would appear attractive.

This standard framework is a necessary starting point for evaluation of changes in size and weight regulations. However, the limitations cited above—that analyses have not been oriented toward attaining defined objectives and have not been well integrated with the processes of regulation and management—have restricted the framework’s usefulness. The following two subsections examine these problems.

Defining Objectives

Truck size and weight regulations are a mechanism for balancing the potential public costs of truck travel against the benefits of lower shipper and carrier costs for freight transportation. The most useful size and

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

weight study would be a structured search for better means of attaining these goals. These means might entail changes in size and weight regulations coordinated with changes in safety regulations, highway design, user fees, or other areas of highway management. Studies confined solely to evaluating changes in size and weight limits will never reveal such opportunities.

Instead of serving as problem-solving exercises—asking how the size and weight regulations can be used as part of a strategy for increasing the benefits of the highway system—evaluations often have appeared directionless, asking instead what would happen if a specific limit were incrementally changed or if a particular industry proposal were put into effect. In contrast, solving the problem of maximizing highway benefits requires starting with a trial solution, discovering its shortcomings through initial evaluation, and then refining the proposal to overcome the shortcomings and come closer to a satisfactory solution. This iterative process, if not entirely lacking in past studies, has seldom been explicit or systematic.

Past studies’ estimates of bridge costs illustrate the importance of aiming for objectives. The past DOT and TRB studies have identified regulatory options that appeared attractive considering freight costs, pavement wear, and truck traffic reduction, but were predicted, according to the conventional cost-estimating method, to generate high costs for replacement of deficient bridges to accommodate the new trucks. This finding usually has been the end of the analysis. In contrast, an objective-oriented approach would examine the problem to see whether there might be some means of reducing bridge costs and at the same time retaining a share of the predicted benefits of the regulatory option. Possible solutions worth exploring would include excluding bridges with high replacement costs and low freight mobility benefits from the network of roads where new trucks would be allowed; adjusting truck dimensions to reduce bridge costs (imposing minimum length requirements, for example, would reduce certain costs); making greater use of retrofit strengthening as an alternative to replacement of bridges; and performing more intensive maintenance and inspection to produce an offsetting reduction in the risk of bridge damage. A similar problem-solving approach in other elements of size and weight studies— including evaluations of safety, productivity, and traffic impacts— would likely reveal more nearly optimal truck size and weight solutions.

This not to say that such an analysis approach would necessarily reveal a basis for justifying the liberalization of regulations. The analysis could very well show (in this example) that none of the innovative

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

approaches to bridge management would reduce costs enough to justify the liberalization in question, or the regulatory change might be ruled out by categories of costs other than bridge costs that proved to be unavoidable.

Because of their orientation toward evaluating the impacts of changing dimensions instead of seeking means of attaining objectives, most past studies have ignored some of the most promising policy alternatives, in particular, performance standards and pricing. Performance standards are regulations that require vehicles to pass specified performance tests demonstrating that they are safe and compatible with the design of the highway system. Pricing policies that set road user fees more nearly equal to the actual costs occasioned by each truck and trip would provide incentives for operating trucks that reduced public as well as private costs. The government would calculate the proper fee to charge for any particular vehicle and trip, and the user would decide whether the benefit justified paying the fee.

Since both of these regulatory approaches depend on inducing operators to innovate in order to reduce the costs of truck transport rather than on dictating vehicle dimensions, they do not fit the assumptions of the traditional evaluation framework. Similarly, policies that would simultaneously optimize highway design and vehicle characteristics, as well as technological fixes for truck stability or enforcement problems, are neglected because seeking means to attain objectives is not part of the study design.

As an example of a definition of objectives for truck regulations, the following are the legislatively defined functions of the National Road Transport Commission (NRTC), an independent body formed by the national and state governments of Australia to coordinate road transport reform (NRTC 2000, 32):

Transport efficiency
  • improve road transport industry efficiency and productivity

  • encourage and facilitate innovation in the industry and its regulation

  • encourage and facilitate technological advancements in the industry, e.g., ITS [intelligent transportation systems]

  • encourage and facilitate continuous improvement in the road transport regulatory environment (e.g., monitoring and updating regulation as necessary)…

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

Improve road safety

Minimize the adverse environmental impacts of road transport

Lower administration costs . . .

The NRTC’s responsibilities include, in addition to truck size and weight, a broad range of safety and environmental regulations. But to the extent that these objectives are applicable to size and weight regulations, they would be appropriate in the United States as well.

As a second example, the following are the objectives set for regulatory changes recommended by the committee that authored the TRB Truck Weight Limits study (TRB 1990a, 228):

  • To select from the various changes in truck weight regulations proposed by industry groups and others the most practical means to realize the productivity benefits of increased truck weights while reducing or eliminating possible adverse effects;

  • To make changes in weight limits that would reduce truck accidents and encourage safety improvements in truck design and operation;

  • To provide mechanisms to match user fees with added costs for pavements and bridges;

  • To promote uniformity in the administration of truck weight regulations;

  • To balance the federal interest in protecting the national investment in the Interstate system and facilitating interstate commerce with the interests of the states in serving the needs of their citizens and industries;

  • To develop proposals that are realistic and feasible and would have a reasonable chance of being implemented.

Objectives for the reform of U.S. federal truck size and weight regulations must be defined by Congress. Objectives that may be inferred from past federal legislation are described in Chapter 1. If Congress had articulated clear and attainable objectives at the outset of DOT’s recent Comprehensive Truck Size and Weight Study, it appears likely that the results would have been more valuable in congressional efforts to resolve policy issues.

Integrating Analysis with Practice

The traditional framework for size and weight studies has not fit well with the nature of decision making on size and weight limits. Experience

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
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has shown that some outcomes of changes in regulations cannot be predicted with great certainty; that changes are political decisions often influenced only marginally by the results of rational analysis; and that the evolution of limits in the direction of allowing larger trucks has continued over many decades, in parallel with the development of the highway system. In the long run, it might be more fruitful to adopt an approach to evaluation and reform of regulations that more openly acknowledged uncertainty at the outset and more carefully monitored the consequences of changes. For example, there will be uncertainty in any prospective evaluation as to whether the safety effects of changes in regulations will be positive or negative. However, one cannot defend as erring on the side of safety a policy of doing nothing because the outcome of changes cannot be predicted if a possibility exists that liberalization of the limits would reduce accident losses. An alternative policy might be to liberalize the limits where the available information indicated a high probability of benefits and to impose positive safety requirements on carriers who chose to take advantage of the new limits. This approach would require rigorous monitoring of outcomes, as well as opportunity for review and modification of the new regulations.

The DOT 2000 study illustrates the risk of overselling the usefulness of prospective analyses of regulatory impacts. In 1994, a DOT official stated the administration position that “any decision to establish national weight standards for the entire [National Highway System] should only be undertaken after thorough safety analysis of all the benefits and costs of such an action to all highway users as well as the economy” (James 1994, 12). The Comprehensive Truck Size and Weight Study was begun at this time. Upon its release 6 years later, the DOT report was a careful and informative factual summary of knowledge, but did not resolve any of the quandaries facing decision makers.

Forecasting models will never be adequate for providing more than plausible indications of how markets and technology will react to changes in regulations, especially in the long run. The reliability of forecasts is limited by some irreducible sources of uncertainty:

  • Changes in the environment, such as physical highway conditions and traffic, will affect costs.

  • The process of writing regulations always entails risks of loop-holes and unintended consequences.

  • Decisions of state and local officials in hundreds of jurisdictions regarding regulation and highway management interact with federal regulations in determining outcomes.

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

In addition, enforcement effectiveness, an important determinant of the outcome of regulatory changes, cannot be forecast unless a systematic and substantial effort is made to collect enforcement data and to evaluate alternative enforcement strategies. As discussed in Chapter 4, monitoring and evaluation of the enforcement of federal requirements are weak today.

Because of these uncertainties, regulation is necessarily a process: the regulatory agency should do the best prior analysis possible, but once regulations have been changed, the consequences should be monitored and adjustments made where necessary. Chances for a positive outcome from a regulatory change can be enhanced by giving users incentives to act in consonance with the public interest through enforcement, user fees, and performance regulation.

Recent history provides examples of the uncertainties of regulatory impact predictions. Changes in regulatory language often elicit unanticipated responses. For example, the 1983 law revising the federal limits contained a complex set of vehicle length provisions (49 USC 3111) that proved to be instrumental in the eventual legal acceptance of 53-ft-long semitrailers on nearly all major roads nationwide. Before the law, 45 ft was the most popular length and 48 ft the greatest length commonly in use; today nearly half of all van semitrailers are 53 ft. This result was not explicitly called for in the act and apparently not the intent of the authors, nor was it foreseen by the TRB study committee that attempted to predict how the law would change vehicle usage (TRB 1986). In contrast, nationwide legalization of twin-trailer combinations, for which the act explicitly provided and which was the most controversial provision regarding truck dimensions, has had only a moderate impact on nationwide use of these vehicles. The TRB study predicted that the share of twin-trailer combinations in nationwide combination truck travel would nearly triple by 1990 as a result of the 1983 law, whereas the actual increase was only about 60 percent (Bureau of the Census 1985, Table 13; Bureau of the Census 1995, Table 13).

As a second example, a study in Ontario examined how the trucking industry had utilized the features of new provincial weight limits introduced in the 1970s to develop a great variety of vehicle configurations for specialized uses, which could not have been predicted at the time the limits were enacted. It was also observed that vehicles with undesirable handling properties had appeared among the new configurations and that most of these, though not all, had been withdrawn by their users once the problems had become known (Agarwal

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

and Billing 1988). This Ontario study is a rarity in being a retrospective examination of the impacts of a regulatory change. The review of past studies presented in this chapter reveals that nearly all the studies are prospective. Historically, there has been almost no systematic effort by governments to monitor the effects of changes in regulations after they occur.

It is clear from such experiences that decisions cannot be based on precise prior knowledge of consequences. In addition to prospective “paper and pencil” policy analyses, other necessary components of the process of regulatory evaluation and revision are

  • Problem-solving research—especially field research to, for example, improve vehicle stability or develop more durable infrastructure designs;

  • Trials or pilots—full-scale, scientifically designed tests of new equipment in commercial use before final regulations are enacted;

  • Monitoring—systematic monitoring of effects on the highway system once regulations have been changed;

  • Adaptation—adjustments to regulations, following orderly and straightforward procedures to improve performance when monitoring shows that objectives are not being met and to respond to changing circumstances; and

  • Opportunity for innovation—incentives for truck operators, truck manufacturers, researchers, states, and others to develop proposals for more effective size and weight rules and for proposals to receive consideration.

Because of the administrative pattern of size and weight regulation that has evolved in the United States, no federal agency has the authority or resources to conduct these essential regulatory support activities. To improve the effectiveness of regulation, it will be necessary to establish an institutional home for these functions, define its objectives, and provide it with sufficient resources. Chapter 3 presents the committee’s proposal for such an arrangement.

SUMMARY OF EVALUATIONS OF PAST STUDIES

This section provides a brief summary and comparison of evaluations of the costs and benefits of changes in truck size and weight regulations in prominent past studies. A strict qualitative comparison of results across studies is not possible because of differences among the studies in definitions, specific regulatory changes examined, time periods of

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

data and projections, and methods of reporting results. Nevertheless, comparisons indicate which categories of impacts are likely to be critical in deciding among alternative regulations. In addition, comparisons narrow the range of apparent uncertainty about the consequences of changes in size and weight regulations. For some categories of impacts, past studies have concurred about the order of magnitude of effects, whereas for other categories the results diverge, suggesting methodological problems in the estimates.

The estimates described in this section and in the remainder of the chapter are taken from evaluations of various truck regulatory options in the following studies:

  • Truck Weight Limits: Issues and Options (TRB 1990a). The present discussion refers primarily to the “Combined TTI HS-20/ Formula B” scenario evaluated in this congressionally mandated TRB study (pp. 196–204). This scenario involves changes in federal weight limits similar to those endorsed by the TRB study committee. The scenario assumes that existing state truck length and route restrictions are unchanged, as are federal axle weight limits. Truck weights are limited only by a new federal bridge formula. Under this formula, carriers are allowed to operate six-axle tractor-semitrailers of up to 89,000 lb and configurations with six axles and two 28-ft trailers of up to 96,000 lb.

  • Turner Proposal study (TRB 1990b). This study predicts the consequences of changes in federal and state regulations that would allow carriers to operate trucks with higher gross weights, and moderately greater length for double trailers, on an extensive network, provided the carriers operated the trucks with lower maximum axle weights than those now allowed in federal regulations. The study predicts that the new configuration most likely to be adopted under the new regulations would be a nine-axle configuration with two 33-ft trailers and a maximum gross vehicle weight of 111,000 lb.

  • Comprehensive Truck Size and Weight Study (DOT 2000). The estimates to which this chapter refers are from three of the regulatory scenarios evaluated in this most recent DOT study:

    • The “North American trade” scenario, in which six-axle tractor semitrailers of up to 97,000 lb and configurations with eight axles and two 33-ft trailers weighing up to 131,000 lb are allowed on the current federally defined National Network (the roads where twin 28-ft trailer combinations are allowed by federal law today).

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×
  • The “LCVs nationwide” scenario, in which double-trailer configurations with two full-sized semitrailers (i.e., turnpike doubles) and weight of up to 148,000 lb and doubles with one full-sized and one short trailer (Rocky Mountain doubles) are allowed on a limited network of roads nationwide consisting mainly of 42,000 mi of Interstate highways. No access for these configurations is allowed on lesser roads; operators are required to couple and uncouple the trailers in staging areas adjacent to major highways. Triple-trailer combinations (three 28.5-ft semitrailers) are allowed on a 65,000-mi network of Interstates and other high-quality roads nationwide with provisions for access to local destinations, and short heavy double-trailer configurations similar to that of the “North American trade” scenario are allowed on 200,000 mi of main roads plus access routes (DOT 2000, Vol. III, III-27).

  • The “triples nationwide” scenario, in which triples are allowed on the 65,000-mi network, and no other new trucks are allowed.

  • Road Work (Small et al. 1989). This Brookings Institution study is included in the comparisons in this section to illustrate a nontraditional approach to truck size and weight control. It also is an example of an objective-oriented, problem-solving approach to policy analysis. The policy options evaluated involve no changes in legal limits. Rather, highway agencies are assumed to charge fees equal to the cost of pavement wear caused by each truck, depending on the total weight of a truck’s axles, the distance it travels, and the construction of the road it is using; agencies are also assumed to adopt construction designs that minimize life-cycle costs, usually building heavier pavements than are now customary. The fees provide incentives for carriers and shippers to adopt equipment and practices that reduce highway transportation costs.

The above studies contain evaluations of numerous options for truck regulations and vehicle configurations other than those listed above. However, the estimates for these options are representative and are relevant to further policy options that are discussed in Chapter 3. The truck configurations evaluated in the TRB and DOT studies include all the trucks that are commonly proposed for more widespread use in the United States: turnpike doubles, Rocky Mountain doubles, triples, short heavy double-trailer configurations (the Turner Proposal study vehicle and similar configurations), and heavy six-axle tractor-

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

semitrailers. All these configurations (see Figure 2-1) are in use in significant numbers in North America today. The policy options or scenarios in these studies all involve adding axles to trucks to better distribute loads or increase the total weight carried in the truck. None except the Turner Proposal study involves changing the federal weight limits for single and tandem axles. The four studies forecast the con

FIGURE 2-1 Illustrative truck configurations in use in the United States.

Note: STAA = Surface Transportation Assistance Act. (Source: DOT 2000.)

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

sequences of specified changes in federal and state regulations (or, in the case of Road Work, user fees) in terms of changes in annual truck-vehicle-miles traveled (VMT), private freight costs, highway agency costs, safety, and delay costs. (No study estimated all of these impacts.)

The Truck Weight Limits study and DOT estimates agree that moderate liberalization of federal standards could yield annual cost savings to shippers on the order of 3 to 6 percent of the costs of heavy truck transportation ($7 billion to $13 billion annually at today’s freight volumes and prices) and reductions in truck traffic volume of several percent as compared with costs and traffic volumes if present regulations continued. The DOT study and both TRB studies agree that even after allowing for increases in traffic that would be stimulated by cost reductions, truck traffic volume would be lower than if the regulations were not changed because the new trucks would be more productive than the ones they replaced. The three studies predict that highway agency pavement construction and maintenance costs would be unlikely to be greatly affected by a change in limits that allowed heavier trucks but did not increase axle weight limits and did not provide incentives for carriers to switch from tandem-axle to single-axle configurations. The TRB studies agree that the costs of highway accidents and congestion probably would change in the same direction as the change in total truck-VMT.

The DOT and TRB studies predict that liberalizing federal weight limits would increase the cost of constructing and maintaining highway bridges. The DOT 2000 study presents estimates showing bridge-related cost increases exceeding shipper savings in most cases analyzed, although it is acknowledged that these costs may be overstated. In fact, past studies have not used appropriate methods for estimating bridge costs, as the next section of this chapter explains.

The options to which the estimates summarized above apply are relatively moderate proposals in that they do not involve expanded geographical use of turnpike doubles, Rocky Mountain doubles, or triples. By comparison, the DOT “LCVs nationwide” scenario may approach the extreme of liberalization that would be physically feasible. The DOT 2000 report (Vol. III, 2-7) describes this scenario as a limiting case rather than as a policy proposal. It projects that nationwide use of LCVs would yield twice the freight cost savings of the “North American trade” scenario with about the same infrastructure costs, and that allowing triples nationwide without expanded use of large double-trailer configurations or any other changes in regulations would yield freight cost savings 50 percent greater than those of the “North American trade” scenario with one-fourth the infrastructure cost increase.

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×

The two TRB committees reached consistent conclusions about safety effects for the changes in vehicle characteristics they considered. The studies evaluate changes affecting heavy single-unit trucks, tractor-semitrailers, and short double-trailer configurations. They conclude that increased use of larger trucks as a result of changes in size and weight limits would have little overall effect on highway safety because small possible increases in accident rates per truck-VMT would be approximately offset by the reduction in truck-VMT resulting from the new trucks’ higher productivity. Accident rates per ton-mile of highway freight are predicted to decline. An earlier TRB study committee that projected the impacts of the twin-trailer combinations authorized by federal law in 1983 reached similar conclusions about systemwide safety impact and net change in truck traffic (TRB 1986). The DOT 2000 study does not estimate safety impacts. However, it presents estimates of accident rates and truck-VMT that are consistent with the TRB committees’ conclusions.

In the estimates in Road Work, operators are predicted to add axles voluntarily, convert to truck configurations that generate lower highway costs, and drive more on Interstates (which have low marginal pavement costs) to reduce their payments of new user fees, which depend on axle weights and on miles and routes driven. Shippers are predicted to shift a small fraction of shipments to rail to avoid higher road user fees. The results are qualitatively similar in many respects to the projections of the effects of liberalized size and weight regulations in the TRB and DOT studies, although Road Work assumes the limits are unchanged. Highway agency costs decline as a result of the combined effects of carriers’ reduction of axle loads and agencies’ adoption of heavier pavement designs that minimize life-cycle costs. User fee revenues decline after equilibrium is reached because of the highway cost savings. Truck traffic volume and shipper costs decline slightly.

Road Work illustrates how size and weight can be controlled with user fees and how agency costs can be controlled by pricing and design optimization. The estimates indicate that it is important to consider combined strategies involving pricing and design as well as modifications in traditional size and weight regulations to find the best method of attaining these objectives.

ESTIMATION METHODS AND RESULTS

The following subsections review results for each of the principal evaluation criteria employed by the past studies:

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×
  • Truck traffic volume and freight costs;

  • Highway pavement costs;

  • Highway bridge and structure costs;

  • Impacts on traffic operations and pollution, which are of primary significance in urban areas;

  • Effects on traffic volume and land use; and

  • Safety.

In summary, the review indicates that for two criteria—pavement costs and traffic impacts—the uncertainties in available estimates are not so great as to hinder evaluations of proposed regulatory changes. Regarding safety, the available information is weak on the relation of accident rates to gross weight for a given configuration. Bridge cost is a critical criterion not well estimated in past studies, but improved estimates may be possible with available information. Finally, the review reveals three criteria—change in truck traffic volume, costs that may arise from motorists stress and discomfort in mixed automobile and truck traffic, and administrative feasibility—that have been inadequately evaluated in past studies, yet may influence the desirability of policy options in practice. Motorist stress and discomfort, a potential impact of changing regulations ignored in past studies, is discussed in Chapter 1. Administrative feasibility is examined in Chapters 3 and 4.

Truck Traffic Volume and Freight Costs

Projections in the past studies of the effect of new regulations on truck traffic have proceeded according to the following steps:

  1. Predict the new truck dimensions and configurations that would become attractive to carriers under the new regulations.

  2. Classify the truck freight market into segments defined by features believed to influence the attractiveness of the new combinations (e.g., freight density, shipment size).

  3. Estimate unit freight costs for the existing and new configurations in each segment.

  4. Estimate the market penetration of the new configurations in each segment on the basis of relative costs and unquantified performance differences (e.g., route restrictiveness, operational problems presented by double trailers for some kinds of applications).

  5. Estimate the change in the volume of truck freight resulting from the redistribution of freight among the modes, changes in shippers’ logistics practices, or other sources caused by the change in truck costs.

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
×
  1. Assign the postregulatory change in truck traffic to road classes as a function of assumed route restrictions on the new vehicles.

Some studies use quantitative, calibrated models to perform at least some of these steps; for example, Road Work estimates an econometric model of carriers’ choices of truck configuration that is analogous to the mode choice models used in transportation planning (Small et al. 1989, 44–51). In other studies, simple cost comparisons or judgment determines assignments of freight to modes and vehicles. Information sources employed to support estimates are historical data on truck mileage by road class, region, and truck type; estimates of the operating costs of various configurations; interviews with carriers, often focusing on the more difficult-to-quantify aspects of equipment selection; analysis of historical patterns of usage of similar equipment (most vehicles evaluated for nationwide use are already in use in some states); estimates of the cost implications of differences between rail and truck transit times and reliability; and econometric estimates of freight demand elasticity (TRB 1986, 98–109; TRB 1990a, 294–303; TRB 1990b, 78–91; DOT 2000, Vol. III, IV-1–IV-34).

To provide a scale for the regulatory impact estimates, some dimensions of the trucking industry are as follows (Bureau of the Census 1998, Table 10; FHWA 1999, Table VM-1; CCJ 2000, 40–44; AAR 2000):

  • Annual combination VMT (1998): 128.2 billion;

  • Annual heavy (three or more axles) single-unit truck-VMT (1998): 10.4 billion;

  • Average operating expenses per VMT, intercity truckload carriers (1999): $1.47;

  • Annual expenditures for operation of large trucks in the United States (including carriers specializing in shipments of less-than-truckload dimensions, which account for about 15 percent of large-truck-VMT and have higher average costs than truckload carriers): $270 billion;

  • Annual operating expenses of U.S. railroads: $40 billion; and

  • Annual VMT of all motor vehicles on U.S. roads: 2.6 trillion.

Also for comparison, the most common configuration of combination vehicle in general use nationwide today is the five-axle tractor-semitrailer, with the semitrailer usually 42 to 53 ft in length, and weighing up to 80,000 lb. The only multitrailer configuration common nationwide is a tractor pulling two 28-ft trailers, with five axles and a maximum weight of 80,000 lb. Longer doubles, triples, and

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heavier tractor-semitrailers are in use in some states (DOT 2000, Vol. II, III-7–III-9).

The various TRB and DOT projections generally agree on the following results:

  • The new vehicles commonly proposed for more widespread application (LCVs, short heavy doubles, and heavier tractor-semitrailers) would serve niche markets instead of becoming the dominant vehicle type (as the five-axle tractor-semitrailer is now). Only a minority of freight is projected to be carried in new configurations. The reasons for this restricted appeal vary from vehicle to vehicle: multitrailer vehicles have operational drawbacks in many applications, and the longest multitrailer combinations would be restricted to small networks in the projections. The heavy, six-axle tractor-semitrailer would have the greatest market potential, but this combination would not be worth the added cost and tare weight in fleets that specialized in low-density freight and in locales where bridge restrictions limited access.

  • In all DOT and TRB projections, allowing larger trucks causes annual truck-VMT to decline. The studies predict that reducing truck costs by liberalizing regulations would increase the volume of highway freight traffic, measured in ton-miles. The only source of increase that is estimated quantitatively is the diversion of freight traffic from rail to truck that would occur as a result of lower truck costs, although the studies address how consideration of sources of increased traffic other than intermodal diversion might influence the results (e.g., Pickrell and Lee 1998; TRB 1990a, 302–303). Diversion is insufficient to offset the reduction in truck-VMT from greater cargo capacity, according to the estimates. The section of this chapter below on effects on traffic volume and land use addresses demand effects through mechanisms other than modal diversion.

  • Cost savings to shippers and carriers are significant but in most cases somewhat modest in magnitude. Savings are several percent of truck freight transportation costs—on the order of several billion dollars annually. The percentage reduction in shipper costs is smaller than the percentage reduction in truck-VMT.

  • The market models used to predict how changing truck costs would affect truck traffic, although based on cost and traffic data and plausible assumptions, probably are not highly reliable. A principal difficulty is assessing the consequences of vehicle characteristics that are not easily converted into cost differences, for example, the problem that carriers report in using double trailers to serve customers’ docks directly (TRB 1990b, 56–62).

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
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Highway Pavement Costs

Most traffic-related pavement wear is caused by heavy trucks. The states spent $7.3 billion on highway resurfacing, restoration, and rehabilitation projects (“RRR projects”) in 1998, 21 percent of total state highway capital expenditures (FHWA 1999, Table SF12A). This class of projects is motivated primarily by the need to repair or replace worn pavement on existing roads (although RRR projects include nonpavement improvements as well). In addition, the requirements of truck traffic affect the cost of pavement construction for new highways. In its 1997 Federal Highway Cost Allocation Study, DOT allocated 77 percent of RRR costs to medium and heavy trucks (DOT 1997, Figure V-3). This allocation of costs depends on some arbitrary assumptions, but gives an indication of the costs of providing pavement for trucks.

If axle weights are not altered, pavement cost per ton-mile of freight will be little affected by a change in the gross vehicle weight limits. Thus, for example, the effect on the pavement of the passage of 1 million axles, each loaded to 18,000 lb, will be nearly independent of the number of axles per truck because the axles act independently of each other. The sole important exception to this rule is that two closely spaced axles (i.e., a tandem axle) cause less wear on flexible (i.e., asphalt) pavement than they would if they were widely spaced. The wear caused by the passage of one axle is quantified for purposes of pavement design and management in terms of the number of repetitions of the axle passage on a new pavement of a specified design that would cause sufficient wear to necessitate replacement or resurfacing of the pavement.

In light of these characteristics of pavement response to loads, past studies have estimated that if gross weight and vehicle length limits are changed but axle weight limits remain unchanged, pavement costs will change only slightly. Thus, for example, the DOT “North American trade” scenario and the Truck Weight Study permit program are predicted to reduce annual highway agency pavement costs by $120 million and $10 million, respectively. The cost impact is the result of three mechanisms that are, according to past estimates, of secondary importance: (1) a change in the limits may cause carriers to alter the distribution of freight between configurations with tandem axles (such as the five-axle tractor-semitrailer) and configurations that carry more of the load on single axles (such as the five-axle twin-trailer configuration), changing wear on flexible pavement; (2) if the proposed change varies by road class, it may alter the distribution of traffic between

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roads with heavy pavements and consequently low average pavement wear costs (e.g., Interstate highways) and roads with light pavements and high average costs (typically secondary roads); and (3) the market response to the change in truck costs caused by changing limits will affect the total volume of highway freight, and pavement costs will change accordingly. The TRB and DOT studies’ projections of slight declines in pavement costs reflect those studies’ projections that the changes in limits they evaluated would have modest effects on the volume of truck freight transportation. However, if the increase in freight traffic in response to the reduction in truck freight rates were great enough, pavement costs would increase.

In contrast, in the two studies in which policy proposals that would more greatly alter axle weight distributions in the direction of lighter average axle loads are evaluated, pavement wear costs are predicted to be substantially reduced. The Turner Proposal study evaluates the effects of offering carriers the option of operating trucks with higher gross weights but lower axle weights than are presently allowed (TRB 1990b, 168). Annual pavement cost savings of $730 million are estimated. In Road Work, all gross weight, axle weight, and length limits are assumed to remain unchanged, but carriers are charged fees that depend on miles traveled, routes, gross weight, and axle configuration, and that provide a financial incentive to switch to configurations with greater numbers of axles. Also (and more significant for pavement costs in the study’s estimates), highway agencies are assumed to build heavier pavements. Agency costs are predicted to decline by nearly $7 billion annually. This savings estimate appears implausibly large considering the annual rate of pavement-related highway expenditures in the study year, but the qualitative result of large pavement cost savings is conceivable. The full savings of heavier pavements probably could be realized only after a period of years, because it would not be feasible to greatly accelerate pavement reconstruction schedules.

Cost Estimation Methods

The four studies reviewed here use three different models of the relation of truck traffic to pavement costs to predict pavement cost impacts. In the DOT and TRB studies, the pavement cost of changing truck size and weight limits is estimated by assuming that the highway agency acts so as to maintain the same average pavement condition after the change by rescheduling the time of the next resurfacing of roads and by changing the design of future resurfacing treatments, following established pavement design methods, to accommodate the

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new traffic mix. Under this assumption, average user costs that are related to pavement condition (e.g., speed, vehicle maintenance, comfort) do not change over the life of the road.

The TRB studies use a model of the relationship between pavement wear and traffic derived from the AASHTO (1986) pavement design method, which is based on data from the American Association of State Highway Officials (AASHO) Road Test, a test of the effect of truck traffic on pavement wear conducted in 1958. This model has two components:

  • Relationships that predict, as functions of pavement structure (primarily material—asphalt or concrete—and thickness), weather, and soil condition, the expected number of passages of an axle of standard weight of 18,000 lb before the pavement surface deteriorates to a specified degree of roughness (the terminal pavement serviceability index, or PSI) as a result of cracking, rutting, and other forms of wear.

  • Relationships for converting axles or axle groups of any weight into an equivalent number of standard axles. In the AASHTO model, the equivalency factor increases approximately as the fourth power of the weight: for example, a 9,000-lb axle is approximately 1/16 of an equivalent single-axle load (ESAL). That is, a pavement that could withstand 1 million passages of the 18,000-lb standard axle before reaching a specified terminal serviceability rating could withstand 16 million passages of a 9,000-lb axle before reaching the same rating.

The steps in the cost estimation in the TRB studies are as follows:

  1. Compute the change in ESALs from the change in traffic.

  2. Compute each road’s new remaining lifetime until it reaches its terminal serviceability and requires its next resurfacing.

  3. Compute the new resurfacing thickness necessary to maintain the specified pavement lifetime under the new loading instead of the previous loading.

  4. Compute the cost of the traffic change: the change in present value of the cost of future resurfacings that results from changing the time and cost of each future resurfacing.

In this method, user cost (i.e., the added time, vehicle maintenance, and fuel costs of traveling on a deteriorated road) can be ignored because the consequence of the practice of always resurfacing at the

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same terminal serviceability rating is that the average user cost over the life of the pavement is unaffected by a change in truck traffic.

The estimates in the DOT 2000 study and Road Work follow a similar logic but use different pavement wear models (that is, different relationships among vehicle traffic characteristics, pavement designs, and pavement wear). The DOT study’s model, the National Pavement Cost Model (NAPCOM), predicts 11 types of pavement distress as functions of traffic and pavement design (instead of simply predicting PSI for rigid and flexible pavements as the TRB studies do), and models a highway agency’s resurfacing decision as a function of all these distress types (DOT 2000, Vol. III, V-13). Road Work estimates new traffic versus road wear relationships using the AASHO Road Test data (instead of using the relationships AASHTO derived from the data as the TRB studies do) and concludes that the relationship between axle weight and pavement wear follows a third-power law rather than a fourth-power law as in the AASHTO model. In all three models, the pavement-wearing effect of an axle increases exponentially with weight, and the number of passages of an axle that a pavement can withstand before failing increases exponentially with pavement thickness. All three appear to yield qualitatively similar results in estimating the relative costs of various truck weights and axle configurations.

The AASHO Road Test did not measure the pavement wear effects of tridem axles (a set of three closely spaced axles). The table of tridem axle equivalency factors in the AASHTO Pavement Design Guide, which was used in the TRB studies to project pavement effects of increased use of tridems, was derived from less definitive sources (AASHTO 1986, MM-2). This gap in the data is important because several prominent proposals for changing size and weight regulations involve extensive use of tridem axles.

The Problem of Optimizing Vehicle and Highway Design

To discover the best combination of policies, including size and weight regulations and highway design and management practices, it is necessary to examine whether changes in prevailing pavement design and management practices coupled with changes in limits could produce greater public benefits, considering road user costs that depend on pavement condition and highway agency pavement costs, as well as other shipper costs that depend on size and weight limits. It is conceivable that a vehicle under evaluation could be predicted to generate high pavement costs given the assumed highway agency practices, but

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that the high costs could be avoided with a relatively low-cost change in pavement design or resurfacing practices. Pavement durability increases exponentially with increasing thickness of pavement; thus a small increment in expenditure for construction or resurfacing can yield a large increase in pavement life.

The TRB Turner Proposal study committee evaluated the heavier pavement option and concluded that its cost-effectiveness depends strongly on the degree of influence of random and external factors (e.g., weather, materials properties, and construction practices) on pavement life. These factors are poorly understood. If they are important enough, then building pavements much heavier than current practice is unattractive economically, and present state highway pavement design practices may be about correct (TRB 1990b, 30). As noted above, Road Work estimates that a combination of heavier pavement designs and user fees that encouraged operators to add axles and to avoid higher-cost roads would greatly reduce pavement wear costs. A trucking industry–sponsored study also concluded that heavier pavements would reduce highway agency costs as well as user costs, although the estimated savings were smaller than in Road Work (TRI 1990, Appendix A).

None of the methods used in these studies to estimate pavement costs explicitly takes into account the relationship of characteristics of truck suspensions and tires to pavement wear. Tire characteristics affecting pavement are subject to regulation; much research has been devoted to the possibility of mitigating pavement wear as well as improving vehicle stability through suspension design. The TRB studies give some attention to how these factors might affect policy recommendations (TRB 1990a, 80–87; TRB 1990b, 171–176). A comprehensive approach to the problem of optimum design for the highway–vehicle system would include determining whether vehicle regulations or user fees should promote certain tire or suspension characteristics.

Finally, none of the studies described here estimates the optimum combination of axle load limits and pavement design; only existing or lower limits are considered. The optimum axle weight limit will depend on bridge as well as pavement costs.

Highway Bridge and Structure Costs

In most past studies, the greatest predicted cost of allowing larger trucks is the cost of replacing bridges deficient for carrying the new heavier loads. These estimates are summarized below. Important methodological shortcomings of the past estimates are then described.

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Finally, a more useful method of estimating the bridge costs of changes in regulations is outlined.

Estimates in Past Studies

The bridge cost estimates in the TRB and DOT studies are as follows:

Regulatory Scenario

Annual Highway Agency Bridge Costs ($ billions)

Annual Agency Plus User Costs ($ billions)

Agency Bridge Costs as % of Freight Cost Savings

Agency Plus User Cost as % of Freight Cost Savings

DOT 2000 “LCVs Nationwide”

4

22

26

154

DOT 2000 “Triples Nationwide”

1

8

5

39

DOT 2000 “NA trade” scenario

5

23

31

159

Turner proposal (TRB 1990b)

0.4

Not estimated

20

Truck Weight Limits permit program (TRB 1990a)

0.9

Not estimated

17

These estimates assume amortization of capital costs at 7 percent annually. (The annualized amounts for the DOT study cases have been computed from the total cost estimates presented in the DOT report.) The estimates in the TRB studies include costs of fatigue damage and of building future bridges to higher design standards, as well as the costs of replacing existing deficient bridges.

Truck Weight Limits presents its estimate with an important qualification: “If all 35,000 additional load-deficient bridges were replaced, total bridge costs would increase by $900 million per year under this scenario. More likely, states would choose to post (rather than replace) many bridges, particularly on low-volume routes” (TRB 1990a, 203– 204). Similarly, the TRB Turner Proposal study recommends bridge management practices the study committee believed would allow states to control bridge costs while avoiding route restrictions that rendered the new trucks unattractive to carriers (TRB 1990b, 206–208). Thus both studies conclude that, rather than actually incurring very high bridge replacement costs, states would or should place restrictions on truck routes and take other steps to limit costs. Similarly, the DOT report notes that in some circumstances, the states could reasonably allow loads exceeding the thresholds for bridge replacement assumed by their cost estimates, post some bridges to bar the larger trucks, and

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strengthen rather than replace some structures; consequently, “certainly not all costs would have to be incurred before heavier loads could be allowed to operate” (DOT 2000, VI-11).

By comparison with the cost estimates, state highway agencies’ capital expenditures for bridge construction and rehabilitation were $4.3 billion in 1998 and $3.7 billion in 1991. State costs for bridge maintenance in 1998 were roughly $1 billion (FHWA 1999, Tables SF4C and SF12A). In addition, local governments spend substantially on bridges, although less than the states.

As the table above shows, the DOT 2000 study estimates that the costs to highway users of delays due to bridge construction necessitated by increased limits would be four to six times the highway agencies’ construction costs. Comparison of the studies’ estimates of bridge costs with freight cost savings shows that these estimates appear to be decisive in judging whether costs exceed benefits for many proposed changes in size and weight regulations.

The method used to estimate bridge costs in the TRB and DOT studies starts with the observation that a bridge is designed to withstand the loadings of the traffic it is expected to bear, and a safety margin is incorporated into the design to allow for the possibility of multiple extreme loads traversing the bridge simultaneously, illegal overloads, uncertainties in material properties, or other unforeseen circumstances. If a bridge is exposed to a single episode of loading significantly in excess of its design load, there is unacceptable risk of irreversible or hazardous damage. Therefore, estimated bridge replacement costs, according to the method used in past studies, depend on the largest loadings to which bridges are predicted to be exposed under the new weight limits and on the load-bearing capacities assumed for the bridges, but not on the frequency of loadings.

The steps in producing the past studies’ estimates are as follows:

  1. Compile a database of bridges on the roads where new trucks will operate, including their load ratings, structural types, span lengths, and traffic volumes. The studies use the DOT-maintained National Bridge Inventory for this purpose.

  2. Specify prototype axle loads and axle spacings of the trucks expected to come into use under the new regulations for each bridge. Also specify loadings imposed by the existing truck types.

  3. Simulate the application of each prototype truck to each bridge, with assumptions about the likelihood of the presence of multiple trucks on the bridge at one time, and compare the estimated forces in

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the bridge structure with some specified acceptable threshold value, expressed as a percentage of the theoretical capacity of the structure. Also, perform this simulation for the existing truck types.

  1. Assign a treatment, either replacement or posting to restrict truck traffic, to each bridge for which any of the loads for existing truck types and for the proposed new trucks cause the force criterion to be exceeded. Strengthening bridges is a third possible treatment, which the TRB and DOT studies do not quantitatively evaluate. The DOT study does not evaluate posting as an option; that is, the highway agency is assumed to replace every bridge for which the load criterion is exceeded.

  2. Estimate the highway agency’s cost for each bridge treatment, primarily, in the TRB and DOT studies, the costs of bridge replacements. The cost of the new regulation is the difference between bridge treatment costs predicted by this method for the existing truck types and costs for the predicted truck characteristics under the new regulations.

In addition to highway agency bridge replacement costs, the DOT 2000 study estimates the cost of traffic delay due to bridge construction. The TRB studies omit this delay cost but include estimates of fatigue cost (i.e., the cost of increased maintenance and loss of useful life resulting from repeated applications of loads) and of the added cost of building new bridges to higher design standards in the future.

Shortcomings of Method of Past Studies

Three deficiencies of the above traditional method of projecting bridge costs limit the usefulness of the estimates thus derived for evaluation of truck size and weight policy. First, the method applies arbitrary criteria to determine whether bridges require replacement. The TRB and DOT studies contain no estimates of how much safety improvement would be gained (e.g., how many bridge failures would be avoided) or of how life-cycle bridge costs would be affected by applying the selected overstress criterion as compared with alternative criteria. Thus, for example, there is no evidence presented in the DOT 2000 study that the additional billions of dollars in user and highway agency costs required to replace bridges exposed to heavier trucks according to the criterion applied by that study, compared with a more lenient standard, would buy any significant public benefit.

The second failing is that past studies generally have not systematically taken into account the possibility of intelligent management of bridge investment and maintenance decisions by highway agencies.

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State highway agencies evaluate bridge replacements individually and try to avoid replacements that have high costs and produce little benefit, especially when alternatives to replacement (strengthening, more intensive inspection and maintenance, or posting) are available. Alternative treatments could produce the same degree of insurance against bridge failure as the extensive bridge replacements projected in past studies at much lower cost to the highway agency and to users.

Finally, consideration of costs other than the highway agency’s bridge replacement costs has been haphazard in past studies. As noted, the TRB studies ignore traffic delay due to construction, while the DOT 2000 study omits fatigue costs and the addition to costs of future bridges. No past study has taken into account the remaining useful life of bridges projected to be replaced under new size and weight regulations, although replacing a structure that would have been replaced for other reasons within a few years has lower net cost than replacing a new structure. The studies also ignore the side benefits of replacing bridges, especially older ones, since new bridges usually are safer and often have increased traffic volume capacity. Because of these deficiencies, the past estimates are not good indicators of either economically justified expenditures or expenditures highway agencies would actually be likely to make if new trucks were introduced.

To gain a better understanding of the significance of the bridge cost estimates of past studies, the committee first examined the results of the DOT 2000 analysis for a few actual bridges and compared those results with the judgments of state bridge engineers. The committee also examined the sensitivity of the estimates to assumptions about the replacement threshold criterion. These investigations are described in the following subsections.

Comparison of Past Study Assumptions with State Practices If state DOTs were actually required to accommodate the larger trucks proposed in the TRB and DOT studies, state bridge engineers would make engineering–economic choices from a range of options including bridge replacement, retrofit reinforcement, posting, or doing nothing when examination showed a bridge had adequate load-bearing capacity. Similarly, a range of practical options is available for reducing travel delays caused by closings. Decisions would be made for each bridge individually and would depend on the volume of car and truck traffic, the remaining life of the bridge, and its functional adequacy.

Cost estimates that do not take into account practical and cost-effective means likely to be employed by the state highway agencies

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to manage the bridge-related costs of accommodating new vehicles will overstate the magnitude of bridge impacts from changing limits and thereby mislead policy makers. As one check on the plausibility of the assumptions of the bridge cost model used in past studies, the committee examined cost estimates for a small number of bridges selected from one state. Results obtained with the DOT study method for determining whether each bridge would require replacement if a particular new truck came into use were then compared with the results of analyses of options for each bridge as bridge engineers of that state would carry out such analyses.

The committee obtained from DOT a list of highway structures in California identified by the bridge analysis method used in the DOT 2000 study as requiring replacement if a specified type of larger truck were to come into use. From the major structures on the list (i.e., those on Interstate highways and those more than 2,000 ft in length), four were selected for analysis, including the two with the lowest average daily traffic volume and two of the three with the highest average daily traffic. The largest is 1.4 mi long and carries 230,000 vehicles a day. Replacing these long structures would incur high construction costs, and replacing those with the greatest traffic volumes would generate high travel delay costs in the DOT estimates. Each of the four structures exceeds the threshold overstress criterion applied in the DOT study under the assumed loading by just a few percent. The four structures were examined by engineers of the state DOT, who reported to the committee that, following its normal practices, the state would not replace, strengthen, or restrict the use of any of the four structures if heavier tractor-semitrailers within the range analyzed in the DOT 2000 study came into use.

Obviously, the small number of bridges selected does not constitute a representative sample. Also, California bridges, which are examined in this and the following subsection, are not necessarily typical of bridge designs and conditions throughout the United States, and practices of the state’s bridge engineers are not necessarily the same as those followed in other states. The purpose of this examination of the state’s bridges is to illustrate some of the problems of estimating the bridge costs associated with changing limits.

The state engineers reported to the committee that, although the state would be unlikely to replace as many structures as the DOT analysis forecast, widespread use of heavier tractor-semitrailers would increase bridge costs to the state, mainly because increased loads would reduce structures’ lifetimes. As noted, the DOT study omits this

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bridge fatigue cost, and the TRB studies (TRB 1990a, 95, 101–102, 203; TRB 1990b, 135, 145–149) estimate that it would be small in comparison with costs related to deficient bridge load-bearing capacity; however, systematic measurements of fatigue costs are lacking. The state engineers’ assessment suggests that fatigue costs are more important than past studies have indicated.

Sensitivity of Cost Estimates to Assumptions To better understand the determinants of bridge cost estimates in the past studies, the committee next asked DOT to provide tabulations of California bridges that would require replacement according to the DOT 2000 study’s method of analysis, and also according to an alternative, more lenient criterion dictating the threshold stress that would trigger replacement of a bridge if a specified larger truck were introduced. The comparison that follows is a sensitivity analysis to show how the selection of this threshold criterion affects costs estimated by using the method of past studies.

AASHTO specifies two alternative criteria for application in determining the loads existing bridges should be allowed to carry. Under the inventory rating criterion, the stress on any structural member of a bridge is not to exceed 55 percent of the yield stress of the member when the bridge is traversed by one truck of the dimensions of interest in each lane simultaneously (on short spans) or multiple trucks per lane (on long spans). Under the operating rating criterion, the stress cannot exceed 75 percent of yield. AASHTO explains the two criteria as follows (AASHTO 1994, 50):

Each highway bridge should be load rated at two levels, Inventory and Operating levels….

The Inventory rating level generally corresponds to the customary design level of stresses but reflects the existing bridge and material conditions with regard to deterioration and loss of section. Load ratings based on the Inventory level allow comparisons with the capacity for new structures and, therefore, result in a live load which can safely utilize an existing structure for an indefinite period of time….

Load ratings based on the Operating rating level generally describe the maximum permissible live load to which the structure may be subjected. Allowing unlimited numbers of vehicles to use the bridge at Operating level may shorten the life of the bridge.

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In applying the rating criteria, the methods used by bridge engineers to calculate stresses on bridges caused by a given loading are conservative, so the actual measured stresses are generally much below calculated stresses (TRB 1990b, 137). States use these criteria for determining which bridges require posting (i.e., restricting use to trucks below the normal maximum weight) and for deciding whether to grant special permits allowing operation of trucks over the normal maximum weight. State practices vary widely, but few states base these decisions solely on the inventory rating (TRB 1990b, 139).

The bridge replacement threshold criteria applied in the DOT 2000 study allow stresses slightly higher than the inventory rating stress on bridges designed to accommodate the HS-20 design load (a standard vehicle defined by AASHTO for use in bridge design) and stresses slightly less than the operating rating on bridges designed to accommodate the H-15 design load. Nearly all Interstate structures were designed to the HS-20 design load or a more rigorous standard; the H-15 design load is typical of older bridges and less important roads. The bridge cost estimates of the TRB studies assume bridges would be replaced or posted if the operating rating were exceeded.

DOT states it chose its criteria because they are “consistent with TS&W [truck size and weight] regulatory practice” (DOT 2000, Vol. III, II-14). The TRB committees apparently decided that the more lenient criterion they applied was reflective of state bridge posting and permitting practices and therefore appropriate for the application. None of the studies defends its chosen criterion with a quantitative argument about economic or safety consequences.

DOT provided the committee with lists of structures on California roads that fail the criterion used in the DOT study and those that fail the criterion used in the earlier TRB studies. These tabulations show a large difference in the number of bridges classified as overstressed by the specified truck according to the alternative criteria: 33 percent of bridges evaluated according to the DOT study criterion versus 6 percent applying the operating rating criterion. The difference in the replacement costs estimated by DOT is even greater: the agency and user costs of replacing the bridges failing the DOT study criterion are more than 30 times the costs of replacing bridges failing the operating rating criterion.

The largest component of replacement costs using the DOT study criterion is the cost of user delay during construction. According to the DOT study criterion, a large number of California structures with average daily traffic (ADT) above 100,000 vehicles are flagged for

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replacement. These high-traffic bridges are the source of the high user costs of replacement. Using the operating rating criterion, very few structures with ADT above 100,000 are identified for replacement.

The structures that fail to meet the DOT study criterion occur throughout all highway systems in California. They include, for example, many Interstate structures. In contrast, nearly all the structures that fail the operating rating criterion are in the minor arterial, collector, and local classes.

In a 1994 congressionally mandated review of the costs and benefits of LCVs, the General Accounting Office (GAO) commented similarly on the great discrepancies in bridge cost estimates produced by differing assumptions about safety margins. In estimates produced by DOT according to GAO’s instructions, agency bridge costs of nationwide use of LCVs on the Interstates ranged from $18 billion using the inventory bridge capacity rating to $1.3 billion using a capacity rating somewhat more conservative than the operating rating. GAO concluded that the latter criterion was reasonable to ensure safety, and that bridge costs could reasonably be reduced even further by judicious exclusions of Interstate segments with high bridge costs from the LCV network (GAO 1994, 24–25).

In another report, GAO comments on the fundamental shortcoming of the bridge cost estimation method used in past studies (GAO 2000, 6). In a review of DOT’s biennial reports to Congress on justifiable levels of funding for the federal-aid highway program, GAO observes:

FHWA’s estimates of total highway and bridge investment requirements in the [reports to Congress] combine estimates derived from the HERS [Highway Economic Requirements System] model, a bridge model, and other types of estimates. The HERS model uses benefit-cost analyses to estimate future highway investment requirements on the basis of information about existing highways. On the other hand, the bridge model is based on engineering data and does not currently use benefit-cost analyses in estimating investment requirements for bridges.

The bridge cost estimating method used in the DOT reports relies on a method for identifying structurally deficient bridges similar to that used for the DOT Truck Size and Weight study. GAO notes (p. 8) that the 1994 Executive Order “Principles for Federal Infrastructure

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Investments” (EO 12893) requires agencies with infrastructure responsibilities to plan for investments using a systematic analysis of expected benefits and costs, and that the bridge cost estimates in the DOT reports to Congress do not appear to be in compliance with the order. The report observes that benefit–cost analysis is superior to the application of engineering standards for determining justifiable highway investments because the latter method selects projects without regard to economic merit.

A study sponsored by the Association of American Railroads also points out the great difference between costs estimated applying the two criteria in the AASHTO standards and criticizes the TRB studies for choosing a criterion that does not reflect the preferences of most state engineers (Harrison et al., 1991). However, this dispute over the relative merits of the alternative criteria is irrelevant to the criticism stated here of the method used for the past studies.

The aim of the above comparisons is not to argue that one criterion or the other is the correct one. The large difference in the costs derived by applying the two criteria indicates that an economic analysis of risks is necessary. Neither criterion is supported by analysis of costs and risks; therefore, it is impossible to say that one or the other criterion is the correct one for evaluating alternative size and weight policies.

A More Realistic Method of Estimating Bridge Costs

As noted above, bridge cost estimates derived by the method of past studies assume replacement of bridges regardless of whether the cost of replacement is justified by the gain in safety and do not fully take into account the capabilities of highway agencies to maintain bridge safety by more cost-effective means than replacing all suspect bridges. Figure 2-2 is a diagram representing a method of estimating the bridge costs associated with changing limits that corrects these deficiencies. In each year, there are several possible outcomes for each bridge on a highway system: failure, replacement, repair, posting, or no highway agency action taken. Each of these possible outcomes has a cost and a probability that it will occur in each year. The probabilities depend on bridge characteristics, traffic, and inspection and maintenance practices. The diagram illustrates conceptually that economic analysis of truck impacts on bridges requires a model that predicts the probability of each outcome as a function of bridge condition and traffic, as well as information about the costs of each outcome. For purposes of national or state-level policy analysis of size and weight regulations,

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FIGURE 2-2 Determining expected bridge costs of changing truck size and weight limits.

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the estimate could be carried out for a sample of manageable size drawn from the population of bridges. The estimate would proceed according to the following steps:

  1. Estimate, from historical data, the expected annual rate of bridge failures on a state’s system under present traffic conditions and management practices. A failure can be defined as the occurrence of damage that would necessitate closing the bridge.

  2. Specify a proposed change in size and weight regulations, and project the resulting change in the distribution of truck loadings on the highway system’s bridges.

  3. Estimate the expected annual rate of bridge failures under the proposed new size and weight regulations.

  4. With this information, estimate benefits and costs for three courses of action:

    1. Do not change the size and weight regulations.

    2. Change the regulations, and eliminate any increase in expected annual failures by replacing bridges.

    3. Change the regulations, and tolerate the new rate of failure. For example, if the present failure rate is 0.1 bridges per year (that is, the expected rate is 1.0 bridge every 10 years), and the rate after the change in limits (with no bridge replacements and no change in bridge management) is projected to be 1.0, the state highway agency might decide that (c) is not acceptable, that is, that the regulations could be changed only if the bridges were upgraded. On the other hand, if the present rate is 0.04 and the projected rate under the new regulations is 0.05, then a case might be made that consideration of the relevant costs (the cost of bridge upgrading, the freight cost savings of higher truck weights, and the costs of a bridge failure) would indicate that the state should decide to adopt policy (c)—accepting the increased risk—although it may be that no state would explicitly adopt such a policy. [Computations in previous truck size and weight studies are done only for policy (b)—the cost of replacing or reconstructing bridges so as to return the risk of failure to the prechange level.]

  1. Knowing the risks would also allow two other possibly valuable strategies to be evaluated:

    1. Change the regulations, and replace selected bridges so that any increase in expected failure rate is partially, but not fully, eliminated.

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  1. Change the regulations, and reduce or eliminate the change in failure rate by means other than bridge replacement, such as more intensive bridge inspection and maintenance or more intensive enforcement of truck weight regulations.

Either strategy (d) or (e) (or a combination) might yield greater benefits than the first options. To see how policy (e) reduces risk, consider as a simple example a structural component with strength R subjected to load S (see Figure 2-3) (Moses 2001, 4–5). The strength of the component and the load to which it may be subject are uncertain, as represented by the probability distributions in Figure 2-3. Because the two distributions overlap, there is a finite probability of failure. If the component is part of a highway bridge, this probability increases over time because the component is subject to deterioration and because trucks tend historically to become heavier (see Figure 2-4). The original level of risk can be restored by either of two methods: the means of R and S can be moved apart (by posting the bridge to reduce truck weights, strengthening members, or replacing the bridge), or the uncertainties in the distributions can be reduced (see Figure 2-5). The de-

FIGURE 2-3 Basic reliability model and failure probability.

(Source: Moses 2001, 5.)

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FIGURE 2-4 Illustration of changing reliabilities over time.

(Source: Moses 2001, 6.)

sired level of risk can be attained by more intensive management (i.e., more frequent inspection and repair of deterioration), which reduces the uncertainty in R, or by more rigorous weight law enforcement, which reduces the uncertainty in S. If, hypothetically, a comparison were made between two highway systems—one system with a rigorous, state-of-the-art bridge inspection and maintenance program within the framework of a comprehensive bridge management system, as well as effective weight enforcement, and with liberal policies regarding allowable vehicle loadings on bridges; and the other system with underfunded and unsystematic bridge inspection and maintenance and lax weight enforcement, and with restrictive bridge loading rules—one could easily imagine that the first system might have the safer bridges and the lower long-run user and highway agency costs.

The method outlined here for estimating the costs of changing size and weight regulations assumes that highway agencies make optimal bridge management and construction decisions. An alternative approach would be to attempt to predict how state bridge engineers would be most likely to behave if the state highway systems were required to accept larger trucks, and to estimate the costs of this behavior. Both kinds of bridge cost estimates would be relevant to the federal policy decision, and with slight modification, the method outlined above could make such a plausible “behavioral” projection.

Cost estimates assuming optimal bridge program decisions would be valuable for three purposes. First, under certain of the federal policy options presented in Chapter 3, possession of minimum bridge

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
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FIGURE 2-5 Relation of uncertainty to reliability: (a) larger safety factors; (b) reduced uncertainties. (Source: Moses 2001, 5.)

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management capabilities would be a precondition for liberalization of federal regulations within a state. Second, state bridge management capabilities are undergoing substantial improvement, as described in the next subsection. Finally, it is vital to understand the nature of the obstacles to realizing the economic benefits of improved truck productivity. If analysis shows that the principal obstacle is inadequate highway management practices rather than physical deficiencies of the highway infrastructure, it will be possible for Congress to recognize and address this inadequacy.

Although the method of past studies sometimes has been described as a behavioral approach, that is, an estimate of how highway agencies would respond if new trucks were introduced given the agencies’ established practices, the projections of the traditional method are unlikely outcomes. It is highly implausible that states would undertake bridge investments of the magnitudes predicted by the method. Instead, some mix of responses would occur: the states would post some bridges, restricting the routes on which the trucks were allowed; some critical bridges probably would be replaced earlier than otherwise; and economic and political pressures might force deviations from precedent so that trucks would be allowed to use bridges from which they might have been barred according to past practice. The consequence of the last response would be the initially invisible costs of higher risk of failure and accelerated deterioration. States with well-managed bridge programs would take action to mitigate these impacts substantially and cost-effectively within budget constraints, replacing selected bridges and rehabilitating or intensifying maintenance on others, while less capable states might do little.

Examples of Applications of Reliability and Risk Analysis

Apparently there is no example of an analysis that has carried out all of the steps in the above method either for evaluating size and weight policy or as part a of a state’s bridge management activities. However, the components of the analysis have been developed and applied, so carrying out the complete analysis would be practicable.

Probabilistic analysis is increasingly recognized as the appropriate basis for bridge design and for cost-effective management of the bridges on a highway system. This approach recognizes that the consequences of highway agencies’ decisions regarding bridge design and maintenance are changes in the risk of bridge failure, and quantifies these changes. The approach treats bridge management as an optimization problem: the optimum program is the schedule of construction, maintenance, and inspection activities that meets a specified objective

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(e.g., a level of bridge failure risk) at lowest cost or obtains the greatest net benefit from a specified budget. Costs are the present value of the expected user and agency costs of failures plus the user and agency costs of bridge maintenance and replacement. Reflecting elements of this overall philosophy, AASHTO has developed bridge design specifications and a manual for evaluation of existing bridges that take account of the statistical variation of loads and of resistance of structural elements (AASHTO 1998; NCHRP 12-48, forthcoming 2002).

One illustration of the practical application of reliability analysis and optimization in bridge management to improve safety is a study employing condition data for a sample of New York bridges. For a fixed maintenance budget, selection of maintenance projects according to risk minimization was estimated to reduce the user costs of failures by 11 percent as compared with costs when projects were selected by traditional criteria on the basis of qualitative condition ratings (Cesare et al. 1993).

The risk-based approach to estimating the bridge costs associated with changing size and weight regulations is more consistent than the method used in past studies with the way bridges are actually affected by increased truck weights and with how highway agencies manage bridges and respond to changes in traffic loadings in practice. Highway agencies’ bridge inspection data reveal how bridges deteriorate over time. Agencies must counteract the deterioration with maintenance, or the bridge will reach a state in which it is judged unfit to carry traffic. For example, a simulation model calibrated with data on New York bridges shows how the expected rate of deterioration of the steel structure of a bridge depends on the frequency of deck joint repairs to prevent water infiltration (see Figure 2-6) (Cesare et al. 1992).

The weight distribution and frequency of truck loadings affect the rate of deterioration. Agencies recognize that loadings are increasing over time and that bridge costs are rising as a result. The costs they recognize are more frequent repair of damaged decks and superstructures and the need for accelerated rehabilitation or replacement to keep structures in service. Agencies typically respond with more intensive maintenance instead of tolerating reduced reliability and increased frequency of failure. The relationship of changes in truck weights to the rate of bridge deterioration and bridge costs was measured in a recent National Cooperative Highway Research Program (NCHRP) project (NCHRP 12-51, forthcoming 2002).

The Ontario provincial highway authority has examined the relationship of repeated heavy loading to bridge fatigue and loss of useful

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FIGURE 2-6 Effect of maintenance on bridge deterioration. (Adapted from M. A. Cesare, J. C. Santamarina, C. J. Turkstra, and E. Vanmarcke, Modeling Bridge Deterioration with Markov Chains, Journal of Transportation Engineering, Vol. 118, No. 6, Nov./Dec. 1992, p. 829. Reproduced with permission of the publisher, the American Society of Civil Engineers.)

life (Dicleli and Bruneau 1995). Ontario’s weight regulations have been among the most liberal in North America, allowing up to 140,000 lb gross weight on eight-axle combinations. Tests of extreme permit loads (up to 290,000 lb) indicated that while typical steel bridges had adequate ultimate capacity to accommodate such overloads, they would be subject to fatigue damage from the cumulative effect of repeated overloads, which would shorten the life of bridge elements. For example, a calculation indicated that adding six passages per day of the 290,000-lb load over a particular bridge would reduce the bridge’s service life by 8 percent. This study demonstrates that the cost of lost useful life of structures caused by increased loadings can be calculated and that bridge users could be assessed these charges on a per-use basis.

Ontario also has a program of bridge testing with the objective of obtaining the maximum economic use of its stock of bridges. A provincial study of bridge testing results reveals that actual load-carrying

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capacities typically differ greatly from those predicted by conventional analytical methods, and that in nearly every case the capacity of the tested bridge was substantially higher than the capacity predicted. The testing program is reported to have saved the province substantial sums of money by allowing it to avoid unjustified bridge replacements. The study includes the caution that “bridges have finite capacities, and the difference between the actual and assumed capacities, however large, cannot be indiscriminately relied upon.” The province regards its active bridge test program as sufficiently ensuring safety (Agarwal and Billing 1988).

A new bridge formula derived from a reliability model was recently proposed in research sponsored by FHWA (Ghosn 2000; Ghosn and Moses 2000). Traffic and bridge characteristics related to load-bearing capacity are described as probability distributions, and a target expected frequency of structural failure is specified. The research shows how to determine truck size and weight limits that meet target levels of risk of bridge failure for a system of bridges. In the study’s reliability model, lifetime risk of failure of critical bridge members is estimated in terms of the principal sources of uncertainty or variability in the capacity of structural elements (for example, measured variability in the capacities of steel structural members as a function of the extent of corrosion) and in the loadings placed upon them from traffic and other sources, including actual variability in vehicle weights and the risk of multiple heavy vehicles occupying a span simultaneously. The research derives a bridge formula that would provide equal failure risk for all steel bridge span lengths, for each of a range of target levels of risk. The study also examines whether trucks satisfying the bridge formula derived for simple-span steel bridges would cause unacceptable failure risks on existing prestressed concrete and reinforced concrete bridges. It concludes that, with few exceptions, the formula would provide adequate protection for these structures as well.

The risk level embodied in the study’s recommended bridge formula is justified on the basis of the consensus of engineering practice. Future extension of such an analysis, following the method outlined in the preceding subsection, should involve selecting an optimum risk level for regulating truck weights on existing bridges on the basis of economic criteria, taking into account the actions states can take to control the uncertainties in loadings and bridge conditions.

State highway agencies are making progress toward having the kinds of databases and analytical capabilities needed to control bridge failure risks through inspection and maintenance practices and to

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evaluate the differences in life-cycle bridge costs for alternative strategies of maintenance, rehabilitation, and replacement. These capabilities allow a highway agency to select the lowest-cost set of practices for accommodating a change in truck weight limits. A bridge management system has two components: a database, maintained through a program of bridge inspection, describing the design characteristics, current condition, and traffic characteristics of all the state’s highway structures; and a set of models or procedures to facilitate programming of bridge maintenance, rehabilitation, and replacement. The models predict next year’s bridge conditions on the basis of this year’s maintenance actions and allow the state to find the minimum-cost schedule of bridge work that meets specified goals relative to safety and traffic service. The models also allow the state to select designs for new structures with the lowest life-cycle costs. Today more than 40 state highway agencies subscribe to the PONTIS computer software, a bridge management system developed by FHWA and made available to the states through AASHTO. The quality of state implementations of PONTIS, in particular the quality of the databases, varies greatly, and bridge management system evaluations still are rarely a primary input to states’ bridge program decisions (Marshall et al. 2000). As bridge management practice evolves, however, the states will have greater knowledge and control than previously of the condition of their bridges, allowing greater safety and reliability as well as cost savings.

The Potential of Retrofitting

Neither the TRB studies nor the DOT 2000 study quantitatively assesses the potential of bridge retrofitting as a means to accommodate greater loads. Retrofit strengthening of bridges to increase load-bearing capacity and earthquake resistance is a technique employed increasingly in state highway programs, although no state is known to have undertaken a program of retrofitting specifically to accommodate larger trucks. A study sponsored by the American Trucking Associations estimates that feasible retrofit strengthening could reduce the number of steel bridges on Interstate highways judged to be overstressed by the commonly proposed larger trucks by 70 to 100 percent, according to standard evaluation criteria, depending on the criterion applied and the truck being evaluated. The fraction of steel bridges requiring retrofit would be from 2 to 30 percent, again depending on the criterion and the truck in question. The authors assert that “retrofits would be inexpensive and would involve only a limited number of bridges but could significantly influence the allowable load capacity of the overall highway network” (Fu et al. 1992, 320–323).

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Summary

If heavier trucks are introduced, highway agencies will incur costs for replacement of bridges, more intensive bridge management and maintenance, and lost useful life of some structures. Construction necessitated by bridge deficiencies will cause highway user delay costs. Competent management would make it possible to maintain bridge safety and service to users at a lower cost than that of the strategy of replacing all nominally deficient bridges.

Because of their methodological shortcomings, the bridge cost estimates of past truck size and weight studies are not very reliable guides to policy. Although the correct analysis remains to be conducted, one can conclude that the bridge cost projections in the DOT 2000 study are almost certainly overestimates of the amount of spending that would be prudent for maintaining bridge safety if truck weights were increased. The DOT study itself acknowledges this limitation. Very high estimates of bridge costs from liberalized regulations are inconsistent with the experience of jurisdictions—in particular Michigan and Ontario—that have opened their roads to use by trucks much heavier than the federal weight limits without experiencing costs of the magnitude estimated. Most important, the DOT estimates ignore the great potential for lower-cost methods of maintaining bridge safety that the states are increasingly capable of applying because of the widespread adoption of bridge management systems.

Future truck size and weight studies should produce bridge cost estimates by predicting changes in expected frequencies of bridge failure caused by changes in size and weight regulations and in highway agency management practices; estimating the costs of increased risk; and comparing alternative methods of reducing risk to find the optimum combination of size and weight limits, bridge replacements and postings, intensity of bridge inspection and maintenance, and truck weight enforcement. This analysis should include assessment of the practicability of the alternative strategies. The most important part of this evaluation would be the estimation of relationships between changes in truck traffic on bridges and changes in rates of deterioration. The same methods should be applied for computing cost-based fees for heavier trucks in state permitting programs.

Impacts on Traffic Operations and Pollution

This section describes available predictions of changes in traffic, air pollution, and noise caused by changes in truck size and weight limits. Traffic, pollution, and noise impacts are considered together be-

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cause they are effects that are particularly important in urban areas and because they depend on the relationship of truck volume and performance characteristics to traffic flow.

Changing truck size and weight limits will affect traffic congestion through four mechanisms:

  • Annual truck-VMT of travel will change as a result of changes in truck capacity and greater use of trucks in response to lower trucking costs.

  • Truck travel will be redistributed among roads if changes in limits are different for different classes of roads.

  • If new trucks are longer or less maneuverable or have less power in relation to their weight, each truck-VMT by the new trucks will cause greater perturbation of traffic than a VMT by the trucks replaced.

  • Changes in truck traffic volume and any resulting changes in congestion will alter the costs of highway travel for other highway users, who in response may change the time, route, or quantity of their highway travel. Nontruck travel will also be affected if changing truck size and weight alters business location decisions.

A related chain of effects will change air pollutant emissions:

  • Truck emissions per truck-VMT will change because the new, larger trucks will in general have greater fuel consumption per mile and because temporal patterns of velocity and acceleration may change, especially if the new trucks have different power-to-weight ratios than old trucks or other drive-train differences.

  • The volume and distribution of truck traffic will change. Total annual truck-VMT may either increase or decrease, depending on whether the volume of freight attracted to truck transport by lower costs is great enough to offset the effect of greater capacity per truck.

  • Changes in truck traffic volumes and characteristics will affect the behavior of drivers of other vehicles. Changes in truck volume and performance will change other drivers’ passing or lane-changing behavior. Changes in the frequency of congested conditions also will alter other vehicles’ temporal patterns of velocity and acceleration. Changes in congestion delay and in velocity and acceleration patterns will alter the emissions of all vehicles.

  • Changes in nontruck travel volume and travel patterns stimulated by the changes in truck traffic characteristics and congestion will affect emissions.

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The following subsections summarize methods and results of the congestion and pollution estimates in the 1981 DOT truck size and weight study, the TRB studies, and the DOT 2000 study. There is more diversity in the studies’ treatment of these costs than for any of the other categories of impacts.

1981 DOT Study

The DOT (1981) truck size and weight study includes an analysis of 14 actual urban road segments, from 1.5 to 13 mi in length, located in states throughout the country. Traffic, population density, and other local data for each segment were obtained and estimates made of changes in traffic, accidents, maintenance costs, congestion, noise, and air quality for each segment for each of the nine regulatory change scenarios evaluated in the study. The case study approach was taken because the authors recognized that urban impacts will be highly variable, depending on local conditions. Presumably the authors concluded on the basis of the case study results that a nationwide urban impacts estimate would not reveal enough new information to be worthwhile.

The estimates were the result of the following computations:

  • Changes in VMT for heavy trucks were predicted by truck configuration. Projections were made regionally, by urban/rural land use, and by highway class. In all the projections, allowing more productive trucks reduced total truck-VMT since the only source of induced truck traffic considered was diversion from rail, which was projected to be too small to offset the effect of increased ton-miles per truck-VMT. Increasing federal limits was projected to increase truck traffic on some roads because trucks would be diverted from state-regulated secondary roads to primary roads subject to federal regulations in response to the liberalized limits. Making limits more restrictive increased total truck-VMT.

  • Passenger car equivalents (PCEs) for the new truck types were estimated. The PCE of a truck is the number of cars that would have to be added to the traffic stream to have the same effect on traffic flow as adding one truck to the stream. The report does not explain how PCE values were selected. Apparently they were assumed in most cases to be the same for new trucks as for the trucks replaced, although the study did include some direct observations of trucks in traffic.

  • Peak and off-peak speeds were predicted to change as a result of the change in total PCE volume on the road segments. Thus speeds in general increased in scenarios involving the liberalization of limits.

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Travel time changes were predicted on the basis of the speed changes. Speeds did not necessarily increase on every road segment in the liberalization scenarios because of diversion of trucks from secondary roads.

  • The change in emissions was predicted on the basis of speed-dependent emissions factors (grams per vehicle-mile) for several classes of vehicles. The documentation available does not state how DOT derived emissions rates for larger trucks. (The study included a separate analysis of nationwide changes in emissions.)

  • Change in noise exposure was predicted on the basis of models of road noise as a function of traffic volume, speed, and vehicle mix and models of truck noise as a function of configuration and speed, as well as data on highway geometry and adjacent land use for the case study segments.

The 1981 study does not include estimates of changes in emissions of particulate matter, which today are regarded as the most harmful diesel emissions (TRB 1996; McCubbin and Delucchi 1999). No economic value is placed on the projected changes in emissions.

Median values of impact estimates among the 14 urban sites in the most extreme higher-weight scenario (Scenario J, which would allow short double-trailer configurations of up to 105,000 lb and tractor-semitrailers of up to 90,000 lb) were as follows:

Impact

Percent Change from Base Case

Peak volume/capacity ratio

-0.15

Off-peak volume/capacity ratio

-0.25

Annual oxides of nitrogen emissions

-0.50

Annual hydrocarbon emissions

-0.30

Population with noise exposure above a specified threshold

+0.80

Thus the projected impacts are all very small and generally, with the exception of noise, are favorable. Because the traffic projections involve redistribution of truck traffic among routes in response to route-specific changes in regulations, some of the effects on the case study roads could be augmented or offset by effects on other nearby roads. Since these estimates were published in 1981, great changes have occurred in traffic, highways, vehicle emission characteristics, and population distribution. If the estimation method of the DOT study were repeated with up-to-date data, the new estimates could differ greatly from those listed above. However, updating might not alter the qualitative finding that

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the percentage changes are small. The most serious deficiency in the estimates may be omission of consideration of particulate emissions.

TRB Studies

Quantitative estimates of changes in congestion delay or emissions resulting from changes in size and weight regulations were not included in the TRB studies (TRB 1986; TRB 1990a; TRB 1990b). Engineering evaluations of performance features of the new trucks and characteristics of their interactions with other vehicles, and the qualitative relation of these items to traffic flow, were involved in the studies. The features considered were as follows:

  • Speed on upgrade;

  • Traction ability;

  • Passing (and being passed) on two-lane highways;

  • Freeway merging, weaving, and lane changing;

  • Freeway exiting maneuvers;

  • Intersection sight distance requirements;

  • Signal timing requirements;

  • Downhill operations;

  • Longitudinal barrier requirements;

  • Splash and spray;

  • Truck blind spots;

  • Blockage of view; and

  • Aerodynamic buffeting.

It was concluded that some of these features of the proposed new trucks would likely have adverse consequences for traffic flow in truck-for-truck comparisons with existing vehicles; that most effects would be small; but that if new trucks were underpowered compared with the vehicles replaced, the effect of poorer ability to maintain speed could be significant (TRB 1990a, 123; TRB 1990b, 110–111). It was predicted that negative effects on traffic would be approximately offset by reduced truck traffic, resulting in a negligible change in aggregate congestion delay.

DOT 2000

The methods used for producing estimates of congestion and pollution impacts in the DOT 2000 study are similar to those used for the 1981 study. Urban case studies were not conducted, but evaluations were carried out for a random sample of road segments so that total nationwide impact estimates could be produced. New estimates of

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PCEs for larger trucks were produced by means of microsimulation traffic models.

Congestion The traffic models FRESIM, NETSIM, and TWOPAS were employed for the PCE estimates. The models were run for a set of road segments intended to be representative of the range of conditions on all U.S. roads, and the results were scaled to national totals using the FHWA Highway Performance Monitoring System (HPMS) sample road segment database. Inputs required for each sample segment were road geometry and traffic volumes, speed distributions, acceleration and deceleration rates, and hill-climbing speeds for each of several vehicle classes. Truck PCEs were estimated by running the models with and without trucks in the traffic stream and comparing speed-flow curves from the runs (DOT 1998).

In model runs, the various existing and proposed new truck configurations were characterized by two parameters: power-to-weight ratio and length. Power determines speed and acceleration, while length determines space requirements in the traffic stream and other vehicles’ passing behavior. For the study’s final estimates of congestion effects, it was assumed that newly introduced larger trucks would have the same power-to-weight ratio as existing trucks when fully loaded. This assumption is supported by examination of trends in power-to-weight ratios, which have been rising in recent years even as average truck weights have been increasing. The study report notes that the largest standard production on-road truck engine models have been getting larger and would be sufficient to maintain the prevailing power-to-weight ratio for any of the new trucks evaluated. The truck operator’s selection of the optimum power-to-weight ratio is an economic decision that depends on customer service demands, safety, and driver preferences; there is no reason to expect this management calculation to change with the introduction of larger trucks. It is also noted that power requirements or minimum speed requirements can be regulated by legislative action.

Under the assumption of constant power-to-weight ratio, estimated PCEs for heavier single-trailer configurations are identical to those for existing tractor semi-trailers, and PCEs for short doubles (for example, the twin 33-ft trailer combination introduced in the DOT “North American trade” scenario) are only slightly greater. Table 2-1 shows selected PCE estimates (DOT 2000, Tables IX-1, IX-2) and ton-mi/PCE-mi (an index of traffic impact per unit of freight).

Thus in the DOT estimates, larger trucks have much greater values of ton-miles per PCE-mile than existing trucks in all conditions.

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TABLE 2-1 Estimates of PCE and Traffic Impact per Unit of Freight

 

PCEa

Traffic Impact (ton-mi per PCE-mi)

Vehicle

Length (ft)

Maximum Payload (1,000 lb)

Congested Urban Arterial

Rural Interstate Level

Rural Interstate 3% Grade

Congested Urban Arterial

Rural Interstate Level

Rural Interstate 3% Grade

Conventional tractorsemitrailer

60

48

3.0

3.3

13.6

8.0

7.3

1.8

“North American trade” tractor-semitrailer

60

62

3.0

3.3

13.6

10.3

9.4

2.3

“North American trade” double

81

93

3.0

3.4

14.1

15.5

13.7

3.3

NOTE: PCE=passenger car equivalents.

aAssumes vehicle weight-to-power ratio of 250 lb/hp.

SOURCE: Derived from estimates in DOT 2000, Tables IX-1 and IX-2.

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Unless introduction of the new trucks caused ton-miles of truck freight to increase substantially, traffic flow would be improved by the introduction of larger trucks. For example, if the DOT study’s “North American trade” regulatory scenario (with a 51,000-lb tridem axle weight limit) were adopted, and every ton-mile of freight diverted from a conventional tractor-semitrailer to a “North American trade” double generated an additional ton-mile of truck freight, traffic flow would hardly be affected because ton-miles per PCE-mile for the “North American trade” double is nearly twice that for the tractor-semitrailers it would be replacing. Such a large volume of induced new freight would be highly improbable. It would imply that a 20 percent drop in the price of truck transportation for the cargoes for which this configuration would be suitable would cause at least a doubling of the volume of those cargoes in trucks, which is much greater than any reported estimate of price sensitivity of truck traffic.

The change in annual hours of delay was computed from the projections of changes in PCE volumes on the sample road segments for peak and off-peak periods. Delay was valued at $13/vehicle-hour. For the “North American trade” scenario, the result is an estimated annual delay cost savings of $3.4 billion.

The study used a queuing model to predict delay caused by bridge construction required to accommodate heavier trucks. The key assumptions were that construction is done on half the structure at a time and that all traffic is funneled onto the open half. (It is not clear how delay in replacing structures on two-lane roads was estimated.) No allowance for diversion to other routes was made. The report presents this cost as a lump sum; in the “North American trade” scenario, it is equivalent to $18.4 billion/year on an annualized basis (at 7 percent), for a net increase in annual congestion delay cost in the scenario of $15 billion.

Air Pollution Nearly all large trucks are powered by diesel engines. Although heavy-duty diesel engines have been subject to increasingly stringent exhaust emission regulations since 1975, emissions of oxides of nitrogen and particulate matter from these engines remain a major concern. Diesels emit other toxic organic compounds as well. These emissions contribute to violations of the National Ambient Air Quality Standards in many urban areas. Research results from the past decade suggest that particulates are a much more serious health risk than has previously been recognized and that diesel particulates may be particularly hazardous because of their size distribution and

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composition. In recent DOT estimates derived from Environmental Protection Agency studies of the economic costs of air pollution, almost all costs of highway-related air pollution are attributed to health effects of particulate matter, and most particulate emissions in the fine sizes that are believed to have the greatest health effect come from heavy-duty diesel-powered vehicles (FHWA 2000). Similarly, a study of air pollution costs per mile of travel for motor vehicles operating in the Los Angeles region estimated an average cost of $0.53/mile for heavy-duty diesel trucks in 1992, 20 times the cost for gasoline cars, under the authors’ baseline assumptions regarding health effects and value of life, with upper and lower bounds of $2.14 and $0.10. The baseline heavy-duty diesel estimate was projected to fall to $0.35 by 2000 as a result of new emission standards. Nearly all the estimated cost is from mortality caused by particulates. The authors observe that “charging this pollution cost would cause a significant change in trucking operations. Presumably, it would also greatly hasten the introduction of new lower-polluting vehicles, thereby lowering the appropriate charges” (Small and Kazimi 1995). Present estimates of the mortality effect of motor vehicle particulate emissions are based on a small number of studies and are controversial. Among the most important questions is the relative importance of exposure to particulates from sources other than tailpipe emissions (Small and Kazimi 1995).

Little information is available on the characteristics of emissions of heavy-duty diesel powered vehicles under actual operating conditions. Present emission models provide no information on the effect of changing weight limits on heavy-duty vehicle emissions. There are almost no data on heavy-duty diesel vehicle emissions at alternative test weights. The sole study identified as addressing this subject (Clark et al. 1999) presents data on emissions for three simulated test weights: 26,000, 36,000, and 46,400 lb. The results show a modest but inconclusive variation in emissions as a function of increasing test weight. Weights in the range of interest for evaluating weight limit changes were not studied. A substantial testing effort would be required to evaluate the emission impacts of the weight changes under consideration.

It is not surprising, considering the lack of data and models, that the DOT 2000 study does not contain pollution cost estimates. However, some of the critical factors in pollution cost projections are the same as those for the study’s congestion projections: power-to-weight ratios of the new trucks and of the trucks replaced, the net change in

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truck traffic volume, and the distribution of the truck traffic volume change across the road system. If new trucks have the same power-to-weight ratio as the trucks replaced, the PCE estimates indicate that traffic flow will be little affected, so it is reasonable to project that automobile emissions would not be affected. However, if, contrary to the DOT projections, the change in limits caused an increase in truck-VMT on congested roads, automobile emissions could increase. The impact on automobile emissions, however, would depend on the magnitude of the speed change caused by the change in truck traffic. If increased congestion reduced speeds to below 20 mph, emission levels for all pollutants would increase. On the other hand, if freeway speeds were reduced from 65 mph to 35 mph, EPA estimates show reduced vehicle emission rates. The increased congestion would depress automobile travel, at least partially offsetting any higher rate of emissions per vehicle-mile on the more congested roads.

Also, with the same power-to-weight ratio, the new trucks would be expected to have temporal patterns of speed and acceleration similar to those of the trucks replaced, and therefore emissions per gallon of fuel consumed would also be similar to those for the trucks replaced. DOT estimates for the 2000 study show that a 40 percent increase in payload weight, the result of increasing gross weight from 80,000 to 100,000 lb, would increase fuel consumption per VMT by about 10 percent (Cohen 1998). In interviews conducted for the TRB Turner Proposal study, carriers reported that a longer combination vehicle’s fuel consumption is 10 to 25 percent greater per VMT than that of a standard tractor-semitrailer in similar operations; consequently, fuel consumption per ton-mile is lower (TRB 1990b, Tables 3-1 and 3-2).

Summary of Cost Estimates

Under the assumptions stated in the preceding section, estimates of the changes in delay and air pollution costs for one of the DOT 2000 regulatory scenarios and one of the TRB studies are as follows:

 

Change in Annual Highway User Delay Costs ($ millions)

 

From Traffic Interaction Effects

From Construction Delay

DOT North American trade (51,000-lb tridem)

–3,400

18,400

Turner trucks

0

1,600

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These estimates are presented as typical of the order of magnitude of impacts implied by the methods of past studies. Under the stated assumptions, introducing larger trucks would be expected to reduce air pollution costs even if total truck freight traffic increased substantially as a result. However, if the new trucks were underpowered or for other reasons greatly perturbed traffic flow, pollution costs could be increased instead of reduced. Congestion delay due to the traffic perturbation caused by large trucks would be little affected, but delays at bridge replacement projects would be a very large cost if such replacements actually proved necessary.

Improved Methods

The greatest shortcoming of the methods used in past studies to estimate congestion and pollution costs has been the oversimplified treatment of the complex interactions between trucks and other vehicles in the traffic stream. Changing the traffic volume, dimensions, and acceleration abilities of trucks will change how motorists drive around them, affecting other vehicles’ patterns of acceleration and braking. Given the predicted changes in these three parameters, traffic and emission impacts could be estimated with a microsimulation traffic model. Such models can estimate changes in vehicles’ temporal patterns of velocity and acceleration in response to a traffic perturbation. These drive-cycle profiles could be used as inputs to a modal emissions model (a model that predicts emissions for a class of vehicles as a function of second-by-second speed, acceleration, and possibly other operating conditions) to predict the change in emissions produced by all vehicles on a road caused by the perturbation.

Microsimulation models are employed regularly in traffic engineering, and the commonly used models have built-in capabilities for estimating emissions as a function of changes in traffic flow characteristics (TRB 2000). Some models have the capability to predict traffic diversion as a function of changing congestion, but any expected changes in the total volume of nontruck travel would have to be supplied as input to the models.

The emission components of existing simulation models are recognized as being in need of updating. These models might have some utility for estimating the change in emissions by automobiles in the traffic stream as a result of a change in truck traffic, but would not at present be capable of estimating changes in emissions by trucks themselves.

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Effects on Traffic Volume and Land Use

Liberalizing truck size and weight limits would reduce the cost of truck freight services and hence increase the quantity of freight carried by trucks. The increase would be the result of diversion of freight from rail and other modes to truck and of the reorganization of production and logistics at existing facilities, substituting transportation for other inputs. Lower freight prices also would affect freight volume by influencing industrial and commercial facility location decisions. Long-run effects would be greater than short-run effects since shippers would have more options in the long run for taking advantage of lower truck costs (see Figure 2-7). The increase in consumption of transportation caused by a reduction in price is sometimes called induced demand (see Figure 2-8). Changes in business location decisions caused by changes in freight costs would eventually have some effect on residential location decisions and personal travel patterns.

FIGURE 2-7 Freight decisions of a firm. (Adapted from Abdelwahab and Sargious 1992; used with permission of the publishers, the London School of Economics and Political Science and the University of Bath.)

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FIGURE 2-8 Induced demand. (Source: Pickrell and Lee 1998.)

Induced demand represents an initial benefit: shippers (or their customers) gain when they choose to purchase more freight services because the price of trucking falls. If highway users paid the full costs of their travel, evaluation of truck size and weight standards would not depend on the magnitude of the change in truck traffic caused by a change in standards. However, the fees a highway user pays do not always match the cost to the highway agency of providing service, and highway travel generates external costs, for example, air pollution and congestion costs that shippers do not take into account when they make freight purchasing decisions. It is only because the prices users pay do not reflect these costs that induced demand must be considered in a full accounting of the costs and benefits of changing truck regulations. If each highway user paid the cost of his or her travel, growth in travel would necessarily add to the general welfare because highway users decide to make additional trips only when the benefits they gain from the trips exceed the cost of the trips to them. However, if users do not pay all costs, and a decrease in the price of transporta-

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tion causes traffic to increase, then some of the additional travel probably will represent a net waste to society because the costs users pay plus the costs they do not pay can be greater than the benefits users derive from the added travel.

In addition to the benefits they derive from the new freight services they choose to buy after the price of truck transportation falls, shippers benefit from lower costs for all freight movements that would have occurred in the absence of the price decrease. Offsetting these benefits of the price decrease are the external costs caused by the induced traffic and the portion of the added infrastructure cost from the induced traffic for which trucks do not pay. If the volume of induced traffic is large enough and external costs and subsidies are great enough, these offsetting costs will be greater than the benefits to shippers, and the net effect of the price decrease will be an economic loss. Conversely, if freight volume is not very sensitive to truck rates and external costs and subsidies are modest compared with prices, the price decrease will cause a net gain. Determining whether the net economic effect is positive or negative is an empirical question that can be answered with information about demand for truck services, external costs, and infrastructure costs.

The recent TRB and DOT truck size and weight studies predict that liberalizing truck size and weight limits would lead to a decrease in annual truck-VMT. These studies most commonly assume that the total volume of freight traffic via all modes is unchanged and that the only source of new truck traffic in response to changing the limits would be freight diverted from rail. The studies either ignore the possibility that new freight traffic will be stimulated by reduced truck costs or argue that such effects would be small.

Projections that liberalized limits would reduce total truck-VMT are important in the past studies’ overall assessments of changes in the limits because they make it possible to conclude that accidents and congestion delays would decrease in total even if these costs were higher per truck-VMT for the larger trucks. Liberalizing the limits thus appears to produce a win–win outcome—lower freight costs and lower public costs of truck traffic—and the studies avoid the sensitive problem of making trade-offs between economic benefits and safety or convenience costs. It is possible that a decrease in truck transportation rates would result in a net benefit to the public even if truck-VMT increased, but if this is the forecast outcome, estimating the magnitudes of costs and benefits and assessing trade-offs becomes much more challenging.

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Although it is reasonable to predict that reducing truck costs will stimulate new freight traffic, present understanding of freight markets does not support reliable prediction of the magnitude or characteristics of the new traffic. Predicting effects on residential land use and personal travel is even more difficult.

Careful consideration needs to be given to the role that should be played by evaluation of expected effects on travel volume and land use in assessments of proposed government policy changes. It is unlikely that model refinements will ever make it possible to predict the effects of transportation policy changes on travel and land use patterns with much confidence. Because of the uncertainty, regulation of freight transportation is unlikely to be a practical tool for managing urban and regional land use and development. What is more, there is no evidence that efforts to control land use, either directly through zoning or indirectly by manipulating transportation costs, are likely to be effective means of mitigating the external congestion, pollution, and accident costs of transportation.

Undesirable land use effects can arise from changes in truck operating costs primarily because shippers and carriers are not held fully responsible for all of the initial costs of truck transportation— accident, infrastructure, congestion, and pollution costs. The practical way to reduce or avoid harmful land use consequences is to eliminate subsidies in the highway program and to control the environmental, safety, and congestion costs of truck traffic through regulation or imposition of fees.

If the public were to decide that it would be desirable to limit the volume of truck freight transportation to promote environmental, safety, or other objectives, tightening truck size and weight limits might be a relatively expensive means of accomplishing this end. It would be desirable to allow trucks to operate at the dimensions that minimized their rate of consumption of resources—that is, the fuel, labor, equipment and infrastructure depreciation, accidents, and pollution costs per ton-mi—if practical means other than dimensional limits were available for restricting traffic volume.

Efficient road user fees would not necessarily lead to great shifts in land use patterns relative to those seen today. Rather, it appears likely that highway users and providers would first seek ways to reduce costs that did not entail major changes in their behavior, for example, through adjustments in trip times and routes, purchase of more fuel-efficient vehicles, and rational investments in highway capacity expansion.

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User fees that internalize all costs for all road users remain a distant prospect. In certain circumstances, however, benefits could be obtained from partial reforms, including better aligning truck user fees with the highway agency’s infrastructure-wear costs for each truck trip. Recent research suggests that such a reformed truck user fee scheme would not have to be sophisticated or complex in order to eliminate a large portion of the inefficiencies of the existing system (Small et al. 1989; TRB 1996). This degree of reform would reduce but not eliminate the risk that size and weight liberalization would have unjustifiable land use impacts.

Safety

The 1941 study by the Interstate Commerce Commission (ICC) of the need for federal regulation of size and weight addressed safety questions to determine whether federal regulation was needed for the sake of safety and whether allowing greater sizes and weights would be compatible with safety. The study report describes engineering analyses of maneuverability, traffic interactions, and braking of large trucks and presents results of a survey of operators’ views on the relation of size and weight to safety. The report summarizes the ICC’s investigation of accident statistics as follows (ICC 1941, 17–19):

The third and potentially most productive approach is through an analysis of the relative accident experience of the various types and sizes of equipment. Analysis… serves, first, to indicate the need for extreme caution in the use of accident rates. Comparisons often made wholly fail to allow for the effect of important variables…. Sizes and weights are only two of the variables to be considered and while their relation to accidents is of primary importance in this investigation, there are other questions of probably much greater importance in the broad field of highway safety.

Second, material presented in the staff report does not conclusively indicate that any greater hazard is associated with commercial vehicles of the larger sizes and weight, considering the conditions under which they are used, than with smaller commercial equipment….

There clearly is need for further study of the complex relations noted above. Such a study could well be made in areas with

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the more restrictive size and weight limits. However, the present analysis has indicated, at least, that there are matters to be watched carefully in the event Federal regulation is undertaken on economic grounds. It also does not clearly show the need for Federal entrance into this field merely for the sake of reducing sizes and weights in the interest of safety.

Studies of federal policy conducted since 1941 have reached conclusions generally similar to the ICC’s cautiously worded statement: available evidence does not show that size and weight, within the range of existing practices, are highly significant safety factors; lack of data may have prevented observation of hazards; and therefore research and monitoring should accompany regulation. It is a source of frustration that 60 years of research has not yielded definitive conclusions on these questions.

Like the ICC study, later policy evaluations, including the DOT and TRB studies, have assembled their assessments of the likely impacts of allowing larger vehicles from multiple kinds of evidence: examinations of the relation of size and weight to vehicle handling and stability and of the relation of size and weight to the interaction of the vehicle with other vehicles in the traffic stream; reported experiences of carriers and drivers using larger vehicles; and statistical studies of accident involvement rates, accident severity, and types and characteristics of accidents. The previous TRB studies all follow the same procedure for estimating the effect of changes in size and weight regulations on accident losses:

  1. Estimate a table of average accident involvement rates (accident involvements per VMT) for trucks by the following dimensions: severity, vehicle characteristics (e.g., weight, configuration), road class, and external conditions (e.g., weather, day/night).

  2. Predict the change in annual VMT resulting from the change in regulations in each cell of the same matrix.

  3. Predict the change in numbers of accidents by severity by summing the product of average involvement rate times change in VMT over all the cells.

The DOT 2000 study presents estimates of accident rates and changes in VMT, but does not carry out step 3 to produce estimates of changes in numbers of accidents.

The above procedure involves an oversimplification because changing truck size and weight may alter traffic conditions, congestion, and

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travelers’ decisions throughout the system in ways that affect accident risk but are not reflected in this standard calculation. For example, accident rates may depend on the level of congestion on a road, and the risk of accidents not directly involving a truck may depend on the density of truck traffic. The method implicitly assumes that if the volume of trucks on a road increases, the accident involvement rate of cars on the road will increase, but this relationship has not been established empirically. It is probably a reasonable approximation (provided the accident rates and changes in truck characteristics and traffic are known), but better understanding is needed of the relationship of accident risk to traffic volume and the mix of vehicles on a road (TRB 1996, 68–72).

The TRB Truck Weight Limits study attempts to simplify the safety risk comparisons for the trucks it considers by estimating two accident involvement rate ratios: the ratio of double-trailer to single-trailer rates and the ratio for a heavy tractor-semitrailer with respect to a conventional tractor-semitrailer. The ratios are intended to reflect the relative risk of the two vehicle types in the same applications. Following this approach, the first two subsections below summarize the evidence for the accident involvement rates, as functions of configuration and of weight, respectively, used in the calculations of changes in accident losses in past studies and the conclusions of those studies about the systemwide safety effects of changes in regulations. The third subsection reviews the conclusions of past studies about the likely safety consequences of changes in vehicle handling, stability, and performance properties that could accompany changes in size and weight regulations. The final subsection presents conclusions.

Relation of Configuration to Accident Risk

Most studies of the relation of configuration to accident risk have presented findings in the form of relative accident involvement rates for single-trailer and multitrailer combinations. Apparently, however, none of these studies incorporates a control for the effect of vehicle weight, so the results reported may reflect the combined effects of configuration and weight. In some studies, double-trailer configurations consistently have higher average weights than the tractor-semitrailers with which they are compared; in others they have lower average weights, depending on the state or region from which the data originate. In general, these studies have confronted four difficulties: the vehicles of interest sometimes represent a very small portion of the traffic stream, so sample sizes may be too small to allow actual differences in accident risk to be measured; data on accident frequency or

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on miles of travel by various vehicle classes are of poor quality; data may lack detail (e.g., truck configuration and weight) required to make the comparisons of interest; and the effects of confounding factors (e.g., road geometry, driver age, time of day) on risk comparisons may be overlooked because of missing data or misspecified models.

The TRB committee that prepared the Twin Trailer Trucks study (TRB 1986) reviewed all available measurements through 1986 of accident rate differences between multitrailer and single-trailer combinations. Of 15 studies, 10 were eliminated because they incorporated insufficient controls, lacked documentation, or had basic methodological deficiencies. The studies reviewed include data on turnpike doubles, but the five studies that were retained all compare twin-trailer combinations with tractor-semitrailers (TRB 1986, 304–329). The TRB committee also reviewed research on relative accident severity for double-trailer and single-trailer combinations and conducted its own analysis of relative severity with a dataset of accidents reported by carriers to the DOT Bureau of Motor Carrier Safety (TRB 1986, 330–348).

In the five studies retained, the ratios of the accident involvement rate of twins to that of tractor-semitrailers (derived from data on various road classes, carrier groups, and degrees of accident severity) ranged from 0.8 to 2.3, with most in the range 0.9 to 1.1 (TRB 1986, 130). The committee concluded from its own analysis of distributions of accident involvement by severity that a lower fraction of accidents involving twins than those involving tractor-semitrailers entailed injury or death; the committee speculated that this result might be the consequence of a higher rate of single-vehicle accidents for twins than for tractor-semitrailers (TRB 1986, 337). The committee acknowledged that no single accident rate study reviewed is fully successful in controlling for the influences of all factors other than configuration that may have affected the measured accident rates (TRB 1986, 4).

Regarding the systemwide safety impact of liberalization of size and weight regulations, in the TRB studies, for the recommended changes (or in the case of Twin Trailer Trucks, for the regulations already enacted), accident involvement rates for combination vehicles per VMT are projected to increase, involvement rates per ton-mile of truck freight are projected to decrease, and total accidents are projected to decrease (TRB 1990a, 12–17; TRB 1990b, 5).

The TRB Twin Trailer Trucks study was conducted shortly after twins had been legalized for use nationwide. The study report (TRB 1986) recommends that DOT, cooperatively with the states, establish

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improved programs for monitoring truck traffic, accidents, and infrastructure costs. In a 1990 report, Data Requirements for Monitoring Truck Safety, another TRB committee recommends a detailed plan for systematic nationwide observation of truck safety (TRB 1990c). However, data programs have not been fundamentally improved, and it is no simpler today, after 20 years of nationwide experience, to compare the accident involvement rates of double- and single-trailer combinations. Several additional special studies have been published since 1990, and their results are generally consistent with the conclusions of the Twin Trailer Trucks committee. The most important of these studies are described below.

The TRB Truck Weight Limits (TRB 1990a, 125–127) and Turner Proposal (TRB 1990b, 120–121) studies update the review of Twin Trailer Trucks by evaluating four more recent studies that used appropriate methodologies to isolate the effects of vehicle configuration on accident involvement rates. The Truck Weight Limits study committee decided to use a value of 1.1 for the ratio of double-trailer to single-trailer fatal and nonfatal accident involvement rates in its estimates of the impacts of changing size and weight regulations, citing the results of one study (Campbell et al. 1988). The Twin Trailer Trucks committee used the same ratio, on the basis of the totality of the research reviewed. The Turner Proposal study committee concluded that the ratio for double-trailer trucks it recommended would be 1.09 if the doubles were equipped with standard A-frame dollies (the dolly is the connector between the tow trailers), but that accident rates would be nearly equal if the doubles were equipped with a type of connector that eliminates one articulation point (the B-train configuration) (TRB 1990b, 221–223). All of the ratios used in these studies are of a magnitude that is offset by productivity gains (i.e., the increase in ton-miles of freight carried per VMT for the larger trucks), so these committees also concluded that the larger trucks they recommended would not be less safe than the trucks replaced per ton-mile of freight services provided.

A 1991 study of the relation of accident involvement rates to configuration in Michigan (Lyles et al. 1991) is noteworthy for several reasons: the state has some of the most liberal size and weight regulations in the United States (including double-trailer combinations weighing up to 164,000 lb); care was taken to collect accurate and detailed data on fatal and nonfatal accidents and truck-VMT; the population of trucks studied is diverse (all Michigan-registered tractors operating on all Michigan roads); and the analysis attempts

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to control for the factors believed to be potentially important influences on accident risk, including road class, time of day, urban and rural conditions, and driver age. Accident data are from state police accident files, and travel data are based on a telephone sample survey of Michigan-registered tractors.

The study revealed no consistent difference between accident involvement rates for single- and double-trailer configurations. In contrast, rates varied greatly by road class: those for non-limited-access highways were typically 2 to 3 times higher than those for limited-access highways, and those for local streets and roads were typically 7 to 10 times higher than those for limited-access highways. Tractors operated by drivers aged 19 to 20 were found to have an accident involvement rate 5 times the average.

The Insurance Institute for Highway Safety conducted two studies in which the case control method was used to the measure the relative accident risk of single- and double-trailer combinations (Stein and Jones 1988; Braver et al. 1997). This method isolates the effects of truck configuration on accident involvement rates from the effects of other factors by comparing the characteristics of trucks involved in accidents with those of trucks observed at the same locations and times of day as the accidents. The 1988 study, with data from selected Interstate road segments in Washington, reports double-trailer involvement rates 2.5 to 3 times those of tractor-semitrailers. The committees that conducted TRB Turner Proposal and Truck Weight Limits studies examined traffic count data for the roads and time periods of the Washington study. They concluded that actual double-trailer traffic volumes were higher than reported in that study and that this apparent undercount was the source of the reported difference in accident rates. The 1997 case control study involved analyzing all combination-vehicle accidents on Indiana Interstates that occurred during a 15-month period. The control data collection method differed from that of the Washington study and was intended to be more reliable but less detailed. The study revealed no increased crash risk for doubles compared with tractor-semitrailers. The authors note that the study design did not allow for control by driver age, and that if drivers of doubles in the data were older or otherwise more competent on average than drivers of tractor-semitrailers, doubles might have a higher accident rate compared with tractor-semitrailers operated by similar drivers.

DOT undertook three analyses in an attempt to produce new accident risk information to support its 2000 Comprehensive Truck Size and Weight Study: a survey that collected accident and travel data from

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carriers that operate longer combination vehicles, an analysis of DOT’s national fatal accident and truck travel databases, and an evaluation of state-maintained truck accident and travel data in states that allow LCVs to operate.

Seventy-five carriers participated in the DOT survey (Ticatch et al. 1996), contributing information on 4,500 accidents during a 5-year period. Accident rates per million VMT computed from the data were 1.79 for non-LCV combinations (tractor-semitrailers and short doubles under 80,000 lb), 1.02 for turnpike doubles, 0.83 for triples, and 0.79 for Rocky Mountain doubles. The difference between the non-LCV and LCV accident rates was statistically significant; the differences among the types of LCVs were not. Fatal crash rates for LCVs and non-LCVs were found to be equal. The authors concluded that differences in patterns of use with respect to road class, time of day, or driver experience could not account for differences between LCV and non-LCV accident rates. The tabulations of usage patterns presented appear to be consistent with this conclusion, but no statistical test of the conclusion was carried out. In particular, while drivers with more experience had fewer accidents, drivers of LCVs had nearly the same professional experience as drivers of non-LCVs, so driver experience cannot account for the difference in LCV/non-LCV accident rates. Some jurisdictions are reported to impose weather restrictions on the use of certain LCVs. The DOT study did not compare accident rates by weather conditions. Differences in weather conditions during operations may explain some part of the difference in LCV/non-LCV accident involvement rates. DOT does not refer to this study in the safety impacts analysis of its 2000 report (Vol. III, Ch. VIII). The study is not conclusive because the data cannot be verified and because statistical analysis was not performed to control for factors other than configuration influencing involvement rate differences. Nonetheless, the conclusions of the carrier survey offer no support for the assertion in the DOT 2000 report (Vol. III, VIII-2) that the apparent lack of evidence of safety problems in studies of LCV accident rates is an artifact of the vehicles’ restricted operating environments.

The analysis of DOT’s national fatal accident and truck travel databases showed the ratio of multitrailer to single-trailer fatal accident involvement rates on all roads nationwide to be 0.97. The ratio ranged from 0.93 to 1.40, depending on road class. A weighted average ratio of 1.11 was computed, with weights assigned to the ratios by road class so as to eliminate the effect of differences between the two configurations in the distribution of travel by road class (DOT

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2000, Vol. III, VIII-1–VIII-4). No measure of uncertainty or test of statistical significance of this ratio was employed. The findings of this DOT analysis contradict those of an earlier study using the national databases, sponsored by the Association of American Railroads, in which higher multitrailer accident involvement rates were reported (Mingo et al. 1991).

The DOT examination of state-maintained accident and travel databases yielded no accident rate estimates. DOT concluded that only one state (Utah) that allows LCVs also maintains the data needed to estimate accident rates by configuration, and that many years of data would be required from this state before a conclusive comparison could be made (DOT 2000, Vol. III, VIII-2).

Relation of Truck Weight to Accident Risk

The Truck Weight Limits study (TRB 1990a) relies on one study (Campbell et al. 1988) for estimates of the relationship between vehicle weight and accident risk for a given vehicle type. The DOT 2000 study makes no statement about the relationship of weight to accident risk. The committee is not aware of any other attempt to measure this relationship.

Truck Weight Limits presents graphs of Campbell et al.’s estimates of fatal accident involvement rates for single-unit and combination trucks by gross weight range and road class (TRB 1990a, 127–131). The conclusion offered is that “these data suggest a moderate increase in accident rates for higher gross weight, although the relatively small number of data points and the high degree of scatter make drawing conclusions from these data difficult” (p. 129). In estimating the impacts of the study’s proposed regulatory changes, it was assumed that a 10 percent increase in gross weight from tractor-semitrailers would increase the accident involvement rate by 2.5 percent for all levels of severity. The mechanism proposed for this connection is that the height of the center of gravity of a truck will increase as the load on the truck increases, and that a higher center of gravity leads to a greater propensity to roll over during turning (TRB 1990a, 108–109).

The TRB report does not include a statistical test of the significance of the perceived connection of accident rate to weight in the Campbell et al. data. The graphs presented appear inconclusive: accident rates rise with weight within some weight ranges on some road classes and fall within others. The data also would not appear to pertain directly to comparison of trucks of different designs and weights, for example, comparison of five-axle tractor-semitrailers at 80,000-lb maximum

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weight and six-axle tractor-semitrailers at higher maximum weight—the two tractor-semitrailers compared in the study’s evaluation of recommended changes in weight regulations.

It is generally recognized that accidents involving large trucks have more severe consequences on average than those involving only smaller vehicles, and that the majority of the fatalities in large-truck crashes are persons other than the truck occupants. Past studies have expressed the concern that allowing truck weights to increase might increase the proportion of truck crashes that result in fatalities. The committees that conducted the Truck Weight Limits study (TRB 1990a, 132–133) and Turner Proposal study (TRB 1990b, 129) concluded that the severity of car–truck crashes would not be affected by an incremental change in truck weights. Direct observational evidence supporting this conclusion is lacking. (As noted in the preceding subsection, the TRB committees did have empirical support for their conclusion that double-trailer accidents are not more severe than single-trailer accidents.) The two study reports cite data showing that the probability that an occupant of a car involved in a crash will be killed is correlated with the change in velocity of the car during the crash. They note further that, in a collision between a car and a truck whose weight is several times that of the car, the change in velocity of the car is largely unaffected by variations in the truck’s weight. Physics also dictates that the energy dissipated in a car–truck collision is insensitive to the weight of the truck if no third object is involved. The finding of Campbell et al. (1988) of a slight or no relationship between weight and fatal accident involvement rate for large trucks is indirect evidence that severity is insensitive to weight.

Relation of Handling, Stability, and Traffic Interaction Effects to Accident Risk

Most past studies, beginning with that of the ICC in 1941, have devoted considerable effort to examining how changing size and weight limits would affect certain dynamic properties and performance characteristics of trucks that are hypothesized to be related to safety. The goals of these examinations have been to understand the physical basis for any observed differences in accident rates among configurations, and to discover ways of redesigning trucks to counteract undesirable changes in dynamic properties brought about by changes in the limits.

Table 2-2, from the Turner Proposal study (TRB 1990b, 98), is an example of these analyses. The left column lists handling and stability characteristics that would be different for the proposed new truck dimensions under evaluation and that, in the committee’s judgment,

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TABLE 2-2 Influence of Variations in Vehicle Parameters on Handling and Stability (Estimates of Turner Study Committee)

 

Parameter

Characteristic

Gross Combination Weight

Cargo Density

Trailer Length

Tires

Suspension

Dollies

Brakes

Low-speed offtracking

Significant

Moderate

High-speed offtracking

Moderate

Significant

Significant

Moderate

Braking efficiency

Significant

Moderate

Moderate

Static rollover threshold

Significant

Significant

Moderate

Moderate

Steering sensitivity

Moderate

Moderate

Significant

Moderate

Moderate

Rearward amplification

Significant

Significant

Significant

Significant

NOTE: Dashes indicate little or no influence. “Significant” indicates that the committee judged variation in the parameter to have a strong effect on value of the handling and stability characteristic. “Moderate” indicates that the variation was judged to have some effect on the handling and stability characteristic.

SOURCE: TRB 1990b, p. 98.

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could be related to accident risk. The other TRB studies and the DOT 2000 study identify similar properties. These characteristics are as follows:

  • Low-speed offtracking—a measure of the displacement that occurs when a combination vehicle makes a turn at low speed (e.g., at an intersection), and its rear wheels follow a path to the inside of the path of the front wheels.

  • High-speed offtracking—a measure of the displacement of the rear wheels to the outside of the path of the front wheels when the combination makes a high-speed turn.

  • Braking efficiency—a measure of brake performance, related to the likelihood that the wheels on any axle will lock during a hard application of the brakes. Wheel lock degrades vehicle controllability during braking and may lead to jackknifing. In addition to controllability during braking, the other dimension of braking performance examined in past studies is stopping distance. The earlier TRB studies (TRB 1990a, 111–112; TRB 1990b, 101) and the DOT 2000 study (Vol. II, V-19–V-20) conclude that the stopping distance of the larger trucks evaluated would not be worse than that of existing trucks. The regulatory changes considered by these studies all involve adding axles to allow increased gross weight, with no increase in maximum axle weights. The studies conclude that the extra axles and extra brakes would allow stopping distance to be maintained. None of the studies evaluates or recommends changes in regulations that would allow existing combination vehicles to carry heavier loads.

  • Rollover threshold—the level of lateral acceleration a truck can withstand before rolling over during turning, a measure of resistance to rollover. For a given truck, rollover threshold decreases (i.e., resistance to rollover is lessened) as the height of the center of gravity of the vehicle and cargo is increased. Since center of gravity will rise as load increases (if cargo density is constant), this performance characteristic often is cited as a potential source of increased accident risk from higher gross weights.

  • Steering sensitivity—if low, implies that a truck requires constant attention and continual steering correction to maintain the driver’s desired path.

  • Rearward amplification—the ratio of the lateral acceleration of the rearmost trailer to that of the tractor during obstacle avoidance or sudden lane changes. Higher rearward amplification means the rear trailer is more likely to roll over during a sudden steering maneuver.

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×

In addition to these handling and stability characteristics, the past studies have compared larger trucks with existing vehicles with respect to certain characteristics affecting interactions with other vehicles in traffic: performance in climbing and descending hills; performance in passing, merging, and freeway exiting maneuvers, and effect on cars performing these maneuvers; time required to cross or turn at intersections; generation of splash and spray on wet roads; truck blind spots and blockage of other drivers’ views; and aerodynamic buffeting of other vehicles. These characteristics could conceivably affect safety and cause delays on congested roads.

Automotive engineers have methods for controlling these behaviors, within limits, by adjustments in vehicle design. For example, changes in suspension and dolly design, in the height of the fifth wheel connection between tractor and trailer, and in the width between the outermost tires can improve resistance to rollover. The FACT truck, a design for a tractor-semitrailer with tank body proposed by two equipment manufactures in 1989, was claimed to have a rollover threshold 25 percent higher than that of existing tankers, although its gross weight was to be 88,000 lb, 10 percent greater than the current federal maximum weight, and its cargo capacity was to be 13 percent greater than that of existing tankers (Klingenberg et al. 1989). Differences in most traffic interaction characteristics between existing and larger trucks could be minimized by properly specifying engines, tires, and brakes. Most promisingly, new technologies, such as electronic braking systems, open up possibilities for greatly reducing objectionable handling and stability behaviors. It remains for the safety benefits of any such design enhancements to be demonstrated as they are developed. These possibilities are described in Chapter 4.

The body of research on handling and stability and on traffic interaction effects that may relate to safety is reviewed in the DOT 2000 study (Vol. III, VIII-6–VIII-13, and Vol. II, V-19–V-28; Fancher and Campbell 1995; Battelle 1995a; Battelle 1995b) and in the earlier TRB studies (TRB 1990a, 103–123; TRB 1990b, 95–119; TRB 1986, 116–127, 270–303). The results of this body of research have been used appropriately to generate hypotheses about the relative accident risks of vehicles. For example, the Truck Weight Limits study concludes (TRB 1990a, 115–116):

Existing five-axle doubles have a unique handling and stability characteristic, namely, rearward amplification of the motion of the lead units, that is not shared by tractor-semitrailers. This

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phenomenon constitutes a negative safety feature of doubles in obstacle-avoidance or sudden lane-changing maneuvers at highway speeds….

For existing five-axle doubles, increased weight would… downgrade the rearward amplification behavior, which may increase the probabilities of rear-trailer overturns during obstacle-avoidance or sudden lane changing maneuvers.

The relationship referred to between weight and rearward amplification has been established through physical measurements and computer simulations using engineering models. However, the relationship between rearward amplification and accident risk is a hypothesis that has not been demonstrated. Few studies have attempted to measure relationships of handling, stability, or other performance properties of trucks to accident risks, and the results of some of the studies that are available do not demonstrate the hypothesized relationship. Therefore, assessments of the physical properties of trucks affecting handling, stability, and traffic interactions cannot be used to produce quantitative estimates of the change in accident losses that would result from a change in size and weight limits. To verify judgments about the linkages among size and weight regulations, vehicle properties, and safety, two kinds of empirically derived relationships would be required: first, a model of how changes in regulations affect the handling, stability, and performance properties of trucks in use; and second, relationships, derived from observation, of accident involvement rates by level of severity as functions of these truck properties. It should be noted that the linkages between size and weight regulations and vehicle handling and stability can be weakened by optimizing vehicle designs. This outcome could be promoted through performance standards, as discussed in Chapter 3.

It is reasonable to assert that prudence dictates minimizing vehicle behaviors such as rearward amplification that appear to entail a risk. However, measurement of the magnitude of the risk related to these vehicle behaviors is essential for cost-effective regulation. If accident risk can be controlled by changes in vehicle design that affect handling and stability, it is important to fully understand and exploit this opportunity, regardless of whether size and weight limits are liberalized. Conversely, it is important to avoid restricting use of vehicles types that do not pose a risk, or attempting to control risk by requiring changes in vehicle design that prove to be ineffective.

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×

In future studies to measure the relation of accident risk to vehicle handling and stability, it will be important to examine experience with nonfatal as well as fatal crashes. Interpretation of studies that estimate fatal rates alone [e.g., Campbell et al. 1988 and the accident rate analysis in the DOT 2000 study (Vol. III, Ch. VIII)] is complicated by a methodological difficulty. These studies confine their analyses to fatal accidents because data on fatal truck accidents are more detailed and reliable than data for other truck accidents, and because fatal crashes account for a very large share of the total costs of truck accidents. However, an analysis that considers fatal crashes alone will be unable to distinguish factors that affect the frequency of crashes from factors that affect the severity of consequences, given that a crash has occurred.

Crash frequency and crash severity are measures of two dimensions of the safety performance of a vehicle. Crash frequency is the number of trucks of a certain type involved in a crash in a time period. The crash involvement rate—the number of trucks of a certain type involved in crashes in a time period divided by the miles of travel by such trucks in the time period—reflects the chance of such trucks being involved in a crash. Crash frequency is thus equal to crash involvement rate times exposure, where exposure is the miles of travel by a category of trucks in a time period. Crash severity refers to the outcome of a crash. Severity is often described by the proportions of crashes in three categories: those causing a death, those causing injury but no death, and those causing property damage but no injury. It follows that fatal crash frequency equals crash frequency times the fraction of all crashes that are fatal. Fatal crash frequency thus mixes the two dimensions of crash frequency and crash severity. A measurement of a change in the fatal crash frequency or fatal crash rate does not tell us how the chance of being in a crash has changed. However, to evaluate the safety significance of such factors as truck stability, off-tracking, and braking, it is necessary to know how they affect the chance of being in a crash, as well as the distribution of outcomes of crashes.

Several of the vehicle performance characteristics hypothesized in past studies to be related to accident risk are correlated with gross vehicle weight. For example, rollover threshold, a vehicle property believed to increase the risk of certain accident types, tends to decrease with increasing vehicle weight for a given truck configuration. Vehicle weight, in turn, is known to be related to the likely severity of crashes in which a vehicle is involved. Therefore, a study of the relation of rollover threshold to accident risk that employed only data on fatal accidents could lead to a mistaken interpretation of the effect of

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weight on severity as an effect of rollover threshold on accident risk. To avoid this confusion, research attempting to measure the relationship between vehicle performance characteristics and accident risk should employ data on accidents of a range of severities and measure the relation of the various characteristics to accident risk and expected severity.

Summary

Studies conducted over the past 20 years have not clearly shown that tractor-semitrailers are a safer means of carrying freight than multitrailer configurations. Past TRB committees that reviewed the research concluded that the safety difference is small. The body of research includes studies that use data for diverse regions, kinds of trucking operations, vehicle configurations, and road environments. The research most commonly reflects experience with the twin-trailer configuration (i.e., shorter double-trailer configurations weighing less than 80,000 lb), but results of studies that include experience for larger doubles are not inconsistent with this general conclusion. The body of past research is inadequate to provide a complete picture of the relative safety of double trailer combinations and tractor-semitrailers. Among the unanswered questions are the relative safety of different sizes of double-trailer combinations, the combined effects of weight and configuration, and the effectiveness of countermeasures. It is important to recognize that any measured differences in accident involvement rates between double trailers and tractor-semitrailers are likely to depend to some degree on specific vehicle characteristics, including the number and spacing of axles and the types of connections between trailers, and that changing these characteristics could change relative accident involvement rates. The FACT truck described above illustrates how vehicles with superficially similar configurations can differ greatly in performance characteristics.

Only one competent U.S. study directly measuring the relationship between weight and accident involvement risk for tractor-semitrailers is available (Campbell et al. 1988). There are some substantial uncertainties in that study’s data, and in any case, a single study of such an important and difficult question is insufficient. The results of that study do not demonstrate a strong relationship between weight and fatal accident involvement rate. The studies on double- versus single-trailer accident rates may provide some support for the finding of the lack of a strong relationship, since the vehicles compared in those studies usually differ in average weight as well as configuration.

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The past TRB studies concluded that differences in accident involvement rates among the truck types evaluated are smaller than the differences in vehicle capacity between the larger vehicles and the vehicles they would replace, so involvement rates per unit of truck freight services would decline. In these studies, therefore, the predicted change in VMT dominates the aggregate safety effect; that is, accident losses decrease in projections in which VMT decreases and vice versa.

The earlier studies’ conclusion that the effect of liberalizing size and weight limits would be to reduce accident losses depends on those studies’ prediction that the change would cause truck-VMT to decrease. If the effect of the change were an increase in total freight shipments (in response to lower truck freight costs) that was great enough to cause truck-VMT to increase, the regulatory change could cause truck accidents to increase even if accident losses per ton-mile of truck freight declined.

Information about the relation of risks to truck characteristics is much weaker than is desirable. The needs include carefully designed and executed statistical measurements of the relation of fatal and nonfatal accident involvement rates to vehicle configuration and dimensions; studies of the relation of vehicle dynamic properties and performance to accident risk; and a model of system-level marginal accident costs, that is, a model of how incremental changes in the volume and characteristics of truck traffic on a network of roads affect accident costs on the network, based on direct measurements of how changes in truck traffic affect the behavior of and risks to car drivers.

Little is known about the effectiveness of the majority of the safety measures recommended by past studies as accompaniments to liberalization of size and weight regulations. In particular, there is little empirical evidence for or against the effectiveness of requiring combination vehicles to meet performance standards regarding handling, stability, and performance in traffic.

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Abbreviations

AAR

Association of American Railroads

AASHTO

American Association of State Highway Officials

CCJ

Commercial Carrier Journal

DOT

U.S. Department of Transportation

FHWA

Federal Highway Administration

GAO

General Accounting Office

ICC

Interstate Commerce Commission

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×

NCHRP

National Cooperative Highway Research Program

NRTC

National Road Transport Commission

TRB

Transportation Research Board

TRI

Trucking Research Institute

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NRTC. 2000. 2000 Annual Report. Melbourne, Australia, Sept. 29.


Pickrell, D., and D. Lee. 1998. Induced Demand for Truck Services from Relaxed Truck Size and Weight Restrictions: Draft. U.S. Department of Transportation, Cambridge, Mass., Oct.


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Stein, H. S., and I. S. Jones. 1988. Crash Involvement of Large Trucks by Configuration: A Case Control Study. American Journal of Public Health, Vol. 78, No. 5.


Ticatch, J. L., M. Kraishan, G. Virostek, and L. Montella. 1996. Accident Rates for Longer Combination Vehicles. FHWA, Oct.

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TRB. 1990b. Special Report 227: New Trucks for Greater Productivity and Less Road Wear: An Evaluation of the Turner Proposal. National Research Council, Washington, D.C.

TRB. 1990c. Special Report 228: Data Requirements for Monitoring Truck Safety. National Research Council, Washington, D.C.

TRB. 1996. Special Report 246: Paying Our Way: Estimating Marginal Social Costs of Freight Transportation. National Research Council, Washington, D.C.

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TRI. 1990. Final Report: Rationalization of Procedures for Highway Cost Allocation. Urban Institute and Sydec, Inc., Oct. 18.

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Suggested Citation:"2. Past Evaluations of Changes in Truck Size and Weight Regulations." Transportation Research Board. 2002. Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267. Washington, DC: The National Academies Press. doi: 10.17226/10382.
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Next: 3. Regulatory Options »
Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles: Special Report 267 Get This Book
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TRB Special Report 267 - Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles recommends the creation of an independent public organization to evaluate the effects of truck traffic, pilot studies of new truck designs, and a change in federal law authorizing states to issue permits for operation of larger trucks on the Interstates.

In 1991, Congress placed a freeze on maximum truck weights and dimensions. Some safety groups were protesting against the safety implications of increased truck size and weight, and the railroads were objecting to the introduction of vehicles they deemed to have an unfair advantage. Railroads, unlike trucking firms, must pay for the capital costs of their infrastructure. The railroads contend that large trucks do not pay sufficient taxes to compensate for the highway damage they cause and the environmental costs they generate. Although Congress apparently hoped it had placed a cap on maximum truck dimensions in 1991, such has not proven to be the case.

Carriers operating under specific conditions have been able to seek and obtain special exceptions from the federal freeze by appealing directly to Congress (without any formal review of the possible consequences), thereby encouraging additional firms to seek similar exceptions. In the Transportation Equity Act for the 21st Century, Congress requested a TRB study to review federal policies on commercial vehicle dimensions.

The committee that undertook the study that resulted in Special Report 267 found that regulatory analyses of the benefits and costs of changes in truck dimensions are hampered by a lack of information. Regulatory decisions on such matters will always entail a degree of risk and uncertainty, but the degree of uncertainty surrounding truck issues is uunusually high and unnecessary. The committee concluded that the uncertainty could be alleviated if procedures were established for carrying out a program oof basic and applied research, and if evaluation and monitoring were permanent components of the administration of trucking regulations.

The committee recommended immediate changes in federal regulations that would allow for a federally supervised permit program. The program would permit the operation of vehicles heavier than would normally be allowed, provided that the changes applied only to vehicles with a maximum weight of 90,000 pounds, double trailer configurations with each trailer up to 33 feet, and an overall weight limit governed by the federal bridge formula. Moreover, enforcement of trucks operating under such a program should be strengthened, and the permits should require that users pay the costs they occasion. States should be free to choose whether to participate in the permit program. Those that elected to do so would be required to have in place a program of bridge management, safety monitoring, enforcement, and cost recovery, overseen by the federal government.

The fundamental problem involved in evaluating proposals for changes in truck dimensions is that their effects can often only be estimated or modeled. The data available for estimating safety consequences in particular are inadequate and probably always will be. Thus, the committee that conducted this study concluded that the resulting analyses usually involve a high degree of uncertainty. What is needed is some way to evaluate potential changes through limited and carefully controlled trials, much as proposed new drugs are tested before being allowed in widespread use.

The committee recommended that a new independent entity be created to work with private industry in evaluating new concepts and recommending changes to regulatory agencies. Limited pilot tests would be required, which would need to be carefully designed to avoid undue risks and ensure proper evaluation. Special vehicles could be allowed to operate under carefully controlled circumstances, just as oversize and overweight vehicles are allowed to operate under special permits in many states. Changes in federal laws and regulations would be required to allow states to issue such permits on an expanded network of highways, under the condition that a rigorous program of monitoring and evaluation be instituted.Special Report 269 Summary

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