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Evaluation of the Turner trucks occurred in the type. Fees might be designed to provide incentives to following categories: safety and traffic, bridges, and choose the most efficient trucks considering both pavements. On the safety issue, the evaluation sug- highway and truck operating costs. The Turner Study gested that adoption of Turner trucks would result in admitted that adjusting taxes by vehicle type would be a modest reduction in truck crashes and a small reduc- difficult under the current taxing structure, but went on tion in truck interference with traffic flow. These to suggest truck tax reform as well as changes in rates. reductions were the result of a decrease in total annual In the final analysis, while the Turner proposal miles of combination truck travel with use of the had merit from the perspective of highway agencies, Turner trucks. For bridges, the Turner trucks would it did not offer sufficient incentives to carriers and require replacement of some 7,000 bridges on Inter- shippers to make it attractive. Even though with the state and primary highways (4 percent of the total). Turner proposal there would be increases in payload This finding represents the major cost to adopting the and perhaps increased profits due to operating larger proposed vehicles. The cost of the bridge replace- and heavier vehicles, the increases in vehicle weight ments at the time (1995) would have amounted to (due to more axles) were not sufficiently offset by $2.8 billion. The additional cost to replace bridges on the additional payload (6 ). non-primary highways would have been $4.1 billion. Other bridge costs associated with this proposal were estimated at $138 million per year once the number CONCLUSIONS of Turner trucks had ramped up to a steady state. This research examined the Mexican truck size If the amount of freight currently hauled by trucks and weight experience with a view to assessing had been diverted in the expected proportions to what may have application within the U.S. context. Turner trucks, a reduction in pavement wear of 33 per- cent would have resulted. On a truck-mile basis, there The research effort was impeded by a lack of accu- would have been a 40-percent reduction in pavement rate data on the number of trucks using the various wear. Considering the greater capacity of Turner highways in Mexico and on their axle loads and trucks, the net effect would have been a 19-percent wheelbases. Actually, a similar situation exists in the reduction in the rate of pavement wear. Savings to United States when trying to rigorously document departments of transportation would have amounted the effects of LCV use. Past efforts at comparing the to $729 million annually once the steady-state period effects of LCVs on safety and infrastructure damage for Turner trucks had been reached. Considering have resulted in inconsistent findings. Furthermore, the pavement savings together with the bridge costs, the costs and other information relevant to determin- adopting this proposal would have reduced annual ing the replacement rate of bridge components and highway agency costs by $326 million, again, once the pavements in Mexico were not available. Information Turner trucks reached full utilization. on enforcement activities targeting large trucks in The Turner study encouraged states to adopt its Mexico was also not well documented, so informa- proposal in its entirety so that the states could real- tion regarding enforcement came entirely from SCT ize the full benefits. Some of the restrictions recom- personnel during meetings with researchers. This mended in order to ensure a high level of safety information indicates that enforcement presence on included minimum and maximum trailer lengths and roadways is relatively sparse. kingpin-to-rear-axle dimensions, antilock brakes on Even though the SCT has installed WIM systems power units, the ability to maintain speed on all grades, at a few locations, WIM data would be of limited and use of the B-train configuration for tank trailers. use in determining the weight characteristics of the Some route restrictions were also included such as vehicles passing over them. Based mostly on inter- avoiding bridges deemed too weak for the expected view information, the authors believe that the WIM loading. Requirements for drivers included a mini- systems are not given the necessary attention to pro- mum of 5 years of employment experience in operat- vide useful data. Critical factors to collecting accurate ing combination trucks and encouragement for drivers WIM data include smooth pavement, adequate main- to complete an accredited training program for the tenance of the WIM system, and frequent calibration. specific truck to be operated. Allowing larger and heavier trucks in Mexico Among other recommendations, adjustments to has not caused significant diversion of freight from highway user fees were proposed to reflect the differ- rail to truck. Rail intermodal service in Mexico is ences in highway maintenance caused by the vehicle provided by private railroad concessionaires, and the 30

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rail intermodal markets in Mexico are well defined future analysis (7 ). Findings of the CTS&W indicated for longer haul distances in which rail intermodal ser- that longer, heavier vehicles would be more produc- vice is more competitive than trucking, even with the tive. According to the CTS&W Study, the following use of LCVs. vehicles result in less pavement wear than the base- Mexico has been regulating large commercial line five-axle tractor-semitrailer: six-axle tractor- vehicles since 1980 when general standards for com- semitrailers, B-trains, and TPDs. Triple trailer LCVs mercial vehicles were first published. Since then, sig- were found to be the least desirable from a pavement nificant changes have occurred including maximum wear perspective. size and weight limits. Many of those changes are The other research effort reinvestigated in this caused by economic or technical changes, but many digest is Special Report 227: New Trucks for Greater others are the consequence of pressure from various Productivity and Less Road Wear: An Evaluation of groups that benefit from larger and heavier trucks. The the Turner Proposal (the Turner Study), which also following is a summary of the findings related to truck predicted productivity gains from adopting the spe- size and weight rules and regulations in Mexico: cific proposed vehicle changes, as well as a modest reduction in truck crashes and a small reduction in Current Mexican truck size and weight lim- truck interference with traffic flow due to a reduction its vary depending on highway classification, in total annual miles of combination truck travel (6). vehicle configuration, and axle configuration. However, the Turner trucks utilize low axle weights, Mexico only allows the longest and heaviest which means that more axles are required to carry a trucks on designated routes. cargo of given mass than if the axle weights were Mexican size and weight regulations also higher. The Turner study recommended adjustments include exceptions and special permits based to highway user fees to reflect differences in highway on highway connectivity facilitation or access maintenance caused by the vehicle type. Fees might to specific industrial or distribution centers. be designed to provide incentives to use the most Maximum gross weight can be increased by efficient trucks, considering both highway and truck 1.5 metric tons for every traction axle and by operating costs. The report admitted that adjusting 1.0 metric ton for every other axle except the taxes by vehicle type would be difficult under the steering axle if the vehicle, driver, and carrier current taxing structure, but went on to suggest truck comply with a number of requisites. tax reform as well as changes in rates (6). There is a special regulation for non-divisible The idea of dedicating heavy-vehicle corridors in loads (NOM-040-SCT-2-1995). For these Mexico has parallels in the United States, but might loads, the maximum GVW is set by the sum require dedicated funding for improving pavements of the maximum axle weights. Depending on and bridges on key routes to accommodate LCVs on the GVW, the carrier might need to request a a limited nationwide network. special permit to use federal highways. Representatives of U.S. border states expressed Axle weights and the bridge formula used in no significant concerns regarding Mexican trucks the United States are more restrictive than the entering their territory with weight issues or non- ones used in Mexico. Mexico uses heavier live complying vehicles mainly because they have loads to design bridges than the United States. installed vehicle safety inspection facilities at most No data were found to indicate whether or not of the border crossings. Some state agencies have adherence to the Mexican bridge formula dur- already developed strategies to reduce the impact of ing construction was monitored or enforced. congestion, air quality, and delay due to cross-border The United States has a long history of research trucking. Some examples are the Port of Brownsville and debate pertaining to vehicle size and weight. This Overweight Corridor Program and the Arizona Over- project reinvestigated two prominent research efforts weight Permit Pilot Program. Currently, there is a dedicated to investigating this issue in years past. One proposal to allow tandem trucks (T3-S2-R4) to cross such effort was the U.S. DOT Comprehensive Truck into the United States. Size and Weight Study (CTS&W Study), a landmark A major contributor to the adoption of heavier research effort that thoroughly investigated various trucks in Mexico is the Mexican Bridge Formula. It aspects of adopting proposed scenarios and devel- is difficult to evaluate this formula in relation to U.S. oped an analytical framework that could be used for Federal Bridge Formula B as there is little or no 31