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Table 16 Maximum allowable GVW for heaviest in particular successes or problems, and it identifies Mexican LCV (T3-S2-R4). constraints that might limit applicability of Mexican practices in the United States. It also examines the Maximum Allowable guidance of recent U.S. studies, such as TRB's Spe- GVW cial Report 227: New Trucks for Greater Productiv- Axle Configuration (Metric Tons) ity and Less Road Wear: An Evaluation of the Turner Proposal (the Turner Study) and the Federal High- One single steering axle 6.5 way Administration's (FHWA's) Comprehensive One tandem power axle 18 Truck Size and Weight Study (CTS&W Study), with Three tandem (non-powered) axles 17 × 3 = 51 respect to the objectives of this task (6, 7 ). Total 75.5 One of the obvious differences between Mexico and the United States that has been a major contribu- tor to the adoption of heavier trucks in Mexico is the Implications of LCV Size and Weight Limits Mexican Bridge Formula. There has been much dis- Pavement and bridge damage caused by commer- cussion in the United States over the past 30-plus years cial vehicles has been discussed in Mexico for several of adopting a more liberal bridge formula than U.S years and was a major topic of discussion during the Federal Bridge Formula B, but all proposals have thus development of the last version of NOM-012. The far been rejected. An example of a vehicle-specific effect of any particular vehicle on the infrastruc- increase that appears to have some momentum ture depends on the maximum allowable axle weight involves allowing a six-axle, 97,000-GVW combina- as well as the axle spatial distribution. tion vehicle (single semitrailer). The maximum allowable GVW in Mexico In Mexico, axle weight limits, gross vehicle depends on the lesser of two values: (1) the sum of weights, and overall length vary as a function of road- the allowances for each axle or axle group or (2) the way type. LCVs in Mexico are restricted to specific maximum weight according to the bridge formula. roadway types and also are required to meet higher The lesser value should be the one that dictates the vehicle performance requirements (see Table 7). maximum GVW. Table 16 shows the limit based on Driver requirements include a special driver's license the axle configuration for the heaviest LCV allowed and additional training. Other requirements include in Mexico (T3-S2-R4). staying within maximum speed limits, keeping head- The result of applying the bridge formula to this lights on for the entire trip, and maintaining a mini- same vehicle configuration results in a maximum mum distance between trucks. These prerequisites are allowable gross weight of 66.5 metric tons. only required for vehicles (LCVs or others) that want According to the criteria for determining maxi- to take advantage of the extra axle weight allowance. mum allowable GVW in Mexico presented above, However, LCVs or "full trailers" gain the most from the maximum GVW for the T3-S2-R4 combination these benefits to increase the gross vehicle weight. should have been 66.5 metric tons because this is the In Mexico, LCVs are restricted by roadway type, lesser value. However, during the development of the but in the United States, they are restricted to a desig- last version of NOM-012, pressure from certain ship- nated network. However, the United States does not pers and carriers pushed the maximum GVW weight currently impose some of the other restrictions. The to be the higher value. On top of that, the SCT suc- research team believes that the United States may cumbed to similar pressure and allowed an additional benefit from investigating the merits of each stipula- 4.5 metric tons until 2013, for a total GVW of 80 met- tion for potential operation of larger/heavier trucks on ric tons. The SCT plans to rescind the additional an expanded LCV network in the United States. 4.5 metric tons for T3-S2-R4 in 2013. Special permits could be considered for the oper- ation of these vehicles and could include stringent driver qualifications and perhaps safety equipment MEXICO/U.S. ANALYSIS beyond that required for current vehicles. Requiring The elements of the Mexican experience that may a nominal fee for the permit would help offset the have relevance for U.S. regulators are summarized administrative costs. Even though tolls are unpopu- herein. The discussion identifies areas where the lar among motor carriers, innovative scenarios could Mexican truck size and weight limits have resulted be investigated to make them more acceptable. For 26

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example, there is technology available that could Limitations in Comparing Mexico allow program administrators to determine the mile- to the United States age of a particular operator on toll roads. This mileage would be treated differently than non-toll mileage by The research team was unable to verify many of the factors that need to be considered to draw conclu- providing reductions in fuel or other taxes to carriers sions and to confidently transfer findings to a U.S. set- using toll roads. This reduction could serve to mini- ting for comparative purposes. For example, accurate mize the burden of "double taxation" that carriers data on the number of trucks using the various high- object to (paying tolls plus fuel and other taxes). ways in Mexico and their axle loads and wheelbases were not available. The cost and other information Successes and Problems relevant to determining the wear rate of bridge com- ponents and pavements were not freely available. This section considers the successes and problems Information on enforcement activities targeting large associated with the Mexican truck size and weight trucks was not well documented; therefore, informa- limits. One of the benefits that comes with allowing tion regarding enforcement came entirely from SCT heavier trucks is that one driver and one vehicle haul personnel during meetings with researchers. more freight per trip, reducing the number of trucks on Even though the SCT has installed WIM systems the highways, assuming maximum load factors. This at a few locations, WIM data would be of limited use benefit can contribute to reducing congestion and fuel in determining the weight characteristics of the use, improving air quality, and reducing the safety risk vehicles passing over these systems. Primarily on incurred by exposure to other vehicles. Finally, larger the basis of interview information, the research payloads would reduce the unit cost of shipping, team believes that the WIM systems are not given which influences international competitiveness and the necessary attention to provide useful data. Other reduces the cost of goods/services to consumers. important factors that are critical to collecting accu- On the subject of problems associated with heav- rate WIM data include smooth pavement upstream ier and longer trucks, a cost to the infrastructure is and downstream of the WIM and adequate mainte- likely to be associated with heavier axle loads, but this nance of the WIM, including frequent calibration. may be partially offset by a reduction in truck travel. WIM systems in remote areas are subject to vandal- For longer vehicles, the impact on the infrastructure ism. Enforcement personnel have other responsibil- is often positive, given that the axle loads are distrib- ities besides enforcement relevant to commercial uted over a longer distance and the reduction in truck vehicles, so their presence on the roadways appears travel is usually greater with LCVs than with heavier to be relatively sparse. vehicles. Pavement condition in Mexico for non- An argument against proliferation of LCVs in the tolled facilities is generally worse than in the United United States has been that larger and more produc- States; however, this may be due to issues of mainte- tive trucks might attract freight that is currently nance or failure to control axle overloading. The hauled by rail to the highway mode. If this were to same is not true of Mexican toll roads since they happen, the net benefit of larger and heavier trucks are newer and have not sustained the magnitude might be diminished in certain areas. In Mexico, pri- of heavy vehicle overloading that non-toll roads in vate railroad concessionaires provide rail intermodal Mexico have. service, which is more competitive than trucking for Longer trucks present an operational challenge long-distance freight. Therefore, there is reason not to with respect to infrastructure geometry, which limits compare the United States to Mexico on the issue of the roads on which they can travel. The SCT requires freight diversion. minimum weight-to-horsepower values for trucks, so their acceleration characteristics in traffic are reason- able. Underpowered trucks would accelerate from Comprehensive Truck Size and Weight Study intersections more slowly and impede other traffic on FHWA's Comprehensive Truck Size and Weight grades. Resulting speed differentials would be unde- Study (CTS&W Study) (7) provides some pertinent sirable, raising the risk of rear-end crashes. Overtak- information with regard to the issues discussed in ing longer trucks on two-lane undivided roadways is this digest. also a potential problem due to increased exposure to The CTS&W Study was similar in many ways to opposing traffic during passing maneuvers. previous truck size and weight studies but unique in 27

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its development of an analytical framework for future River and on turnpikes in five states east of the evaluations of new and different scenarios. It devel- Mississippi at the time the study was published. oped five scenarios to assess the impacts of changes This scenario assumes that LCVs would oper- in truck size and weight. The following describes the ate on a nationwide network of highways. basics of each scenario (7 ): TPDs would consist of two 53-ft trailers weigh- ing up to 148,000 lb; RMDs would consist of · Uniformity Scenario. This scenario required one 53-ft trailer followed by a 28.5-ft trailer all states to adopt federal weight limits on all with a GVW of up to 120,000 lb; and triple- National Network (NN) highways. States that trailer combinations would consist of three exercised grandfather rights to allow heav- 28.5-ft trailers with GVWs of up to 132,000 lb. ier vehicles on the Interstate system would A fourth LCV would have two 33-ft trailers, have to roll them back to the current federal a total of eight axles, and would weigh up to limits. This would force LCVs to operate at an 124,000 lb. 80,000-lb GVW, making them impractical for · HR 551 Scenario. The Safe Highways and all but the lightest loads. Infrastructure Preservation Act (HR 551) had · North American Trade Scenario. This three provisions related to truck size and weight scenario would allow a heavier GVW on cer- limits. It would phase out trailers longer than tain configurations by increasing tridem-axle 53 ft, freeze state grandfather rights, and freeze loads to make them more consistent with loads weight limits on non-Interstate portions of the allowed for tridems in Canada and Mexico. National Highway System. Two alternatives were tested--the first at · Triples Nationwide Scenario. This scenario 44,000 lb and the second at 51,000 lb. At the assumed the operation of triple-trailer combina- higher limit, six-axle tractor-semitrailers would tions across the country at the same weights and be allowed a GVW of 97,000 lb, which would dimensions as the LCVs Nationwide Scenario. allow transporting international containers loaded to the International Standards Organiza- Table 17 provides an insightful comparison from tion (ISO) limit. the CTS&W Study indicating productivity by vehicle · LCVs Nationwide Scenario. LCVs were type by using load equivalency factors per weight of operating in 16 states west of the Mississippi payload (7 ). The measure of pavement damage is Table 17 Theoretical load equivalency factors per 100,000 lb of payload. Load Equivalency Factorsc No. of Rigid Flexible Pavement Vehicles Pavement (5-in Wearing per Fatigue Surface) GVW Empty Payload 100,000-lb (10 in Configuration (lb) Wt. (lb) Wt. (lb) Payload Thick) Fatigue Rutting 5-Axle Semitrailer 80,000 30,500 49,500 2.02 5.7 9.3 10.3 6-Axle Semitrailer 90,000 31,500 58,500 1.71 3.8 7.5 9.6 97,000 31,500 65,500 1.53 4.1 8.4 9.2 B-Train (8 Axles) 124,000 38,700 85,300 1.17 3.9 7.0 7.6 131,000 38,700 92,300 1.08 4.1 7.7 7.5 RMD (9 Axles) 120,000 43,000 77,000 1.30 7.8 9.9 9.5 TPD (9 Axles) 148,000 46,700 101,300 0.99 5.0 7.7 7.2 Triple (7 Axles) 114,000a 44,500 69,500 1.44 8.6 9.8 9.6 132,000b 44,500 87,500 1.14 11.6 11.8 9.0 aLess than truckload. bTruckload. cBased on 18,000-lb single axle with dual tires. SOURCE: Adapted from Comprehensive Truck Size and Weight Study (7 ). 28

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that caused by each vehicle at the maximum weight at necessarily transferrable to other geographic areas which it can operate. As the values indicate, the pave- or roadway types. ment damage varies by type of pavement, specific For these reasons, the U.S. Department of Trans- vehicle type, and the weight at which trucks are portation elected not to predict changes in crash rates allowed to operate. Comparing proposed vehicles in the CTS&W Study (7). The study did, however, against the five-axle tractor-semitrailer indicates gains recommend evaluation of the stability and control or losses related to pavement damage for the assumed properties of different configurations, along with per- pavement types and thicknesses. The following ceptions of drivers. vehicles result in less pavement wear than the baseline five-axle tractor-semitrailer (shaded row in Table 17): Special Report 227: New Trucks for six-axle tractor-semitrailer, B-train, and TPD. Greater Productivity and Less Road Wear: The CTS&W Study also evaluated bridges and the An Evaluation of the Turner Proposal effects of allowing longer and heavier vehicles (7 ). The two most typical bridge designs are HS-20, which Special Report 227: New Trucks for Greater Pro- is common for higher-class roadways, and H-15, ductivity and Less Road Wear: An Evaluation of the which is typical of bridges on lower-class roadways. Turner Proposal (the Turner study) is another study The bridge formula is intended to maintain stress that is relevant to the issues discussed herein (6 ). levels on bridges designed for HS-20 loadings to no In 1984, Francis Turner, a former FHWA admin- more than 5 percent above the design stress. Simi- istrator, made an address to the American Associa- larly, the bridge formula should maintain stress levels tion of State Highway and Transportation Officials on bridges designed with H-15 loadings to no more (AASHTO) in which he advocated a new approach than 30 percent over the design stress. The CTS&W to truck size and weight regulation. This approach, study assumed that when the proposed vehicles met involving trucks with lower axle weights but higher or exceeded these stress levels, new bridges would be gross weights than allowed for the current vehi- cles, came to be known as the "Turner Proposal." required. In reality, some of these bridges could be AASHTO requested that the Transportation Research strengthened and forego replacement. Board (TRB) convene an oversight committee to A significant cost with some of the LCVs was undertake a comprehensive study of the approach associated with road geometric improvements needed and advise the states on the results. to accommodate these proposed vehicles. Geometric Of the many new truck configurations that were deficiencies were primarily interchanges and intersec- possible to investigate, the Turner approach evalua- tions that could not accommodate the longer vehicles tion chose the following four prototypes (6 ): with their increased off-tracking characteristics. The analysis assumed no encroachment on shoulders or · A seven-axle tractor-semitrailer combination adjacent lanes except for at-grade intersections where with a GVW of 91,000 lb and length of 60 ft vehicles could encroach on one lane in the same direc- · A nine-axle double trailer combination with tion of travel. The costs also included staging areas for two 33-ft trailers, a 114,000-lb GVW, and 81 ft certain LCVs at key points along the roadway. In real- overall length ity, the number of staging areas needed would proba- · A nine-axle B-train double with similar dimen- bly be significantly lower than that estimated by the sions as the preceding prototype but with dif- analysis. ferent coupling between the trailers The safety of larger and heavier vehicles has been · An 11-axle double trailer combination with a perhaps the most contentious issue due to the difficul- GVW of up to 141,000 lb ties involved in conducting an analysis of the proposed The axle and axle group weight limits for the vehicles. The safety analysis in the CTS&W Study Turner approach vehicles would be as follows: included several caveats indicating the uncertainties single-axle maximum 15,000 lb (compared to the in attempting to establish safety performance (7). current maximum of 20,000 lb) and tandem axle Reasons for this difficulty are that the weights and maximum of 25,000 lb (compared to 34,000 lb). dimensions of vehicles involved in crashes are usu- These axle loads are extremely low by international ally not known, the vehicle miles traveled (VMTs) standards and are impractical. The TRB oversight of specific vehicle types are not well documented, committee recommended some modifications includ- and crash rates for LCVs currently operating are not ing eliminating the 11-axle prototype. 29