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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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: One or more alternative sets of size and weight limits are specified. 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. 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. 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. 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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. REFERENCES 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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Regulation of Weights, Lengths, and Widths of Commercial Motor Vechicles: Special Report 267 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. TRB. 2000. Transportation Research Circular E-C014: Traffic Analysis Software Tools. National Research Council, Washington, D.C., Sept. TRI. 1990. Final Report: Rationalization of Procedures for Highway Cost Allocation. Urban Institute and Sydec, Inc., Oct. 18.
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