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Federal Highway Administration > Publications > Public Roads > Vol. 64 · No. 5 > DOT's Comprehensive Truck Size and Weight Study - A Summary

March/April 2001
Vol. 64 · No. 5

DOT's Comprehensive Truck Size and Weight Study - A Summary

by James W. March

In August 2000, the U.S. Department of Transportation (DOT) submitted its report on the Comprehensive Truck Size and Weight Study to Congress. This report presented the results of a comprehensive examination of issues surrounding the current federal truck size and weight (TS&W) limits and the potential impacts of changes to those limits. It was the first comprehensive TS&W study by the department since 1981.

Several studies on various aspects of TS&W regulations have been conducted by DOT, the Transportation Research Board (TRB), and others over the years. These studies have highlighted the diverse opinions among states, shippers, carriers, and various other interested groups about the need for changes in federal TS&W regulations.

While several recent TS&W studies have generally included options to both increase and decrease federal TS&W limits, attention has focused primarily on the options to improve productivity through various increases in TS&W limits. Virtually all previous TS&W studies have shown large reductions in shipping costs associated with increases in TS&W limits. The magnitude of cost reductions, of course, has depended on specific assumptions concerning allowable vehicle weights and dimensions and the extent of the road system upon which larger vehicles would be allowed to operate.

Past studies have also noted a variety of potential adverse impacts of increasing federal TS&W limits, including added infrastructure costs, financial impacts on competing railroads, disruption of traffic flow, and potential adverse impacts on safety.

Safety has been one of the issues of greatest concern in previous TS&W studies. Motorists are keenly aware of the growing number of trucks on the road, and many express discomfort when driving in traffic with many large trucks. It has been particularly difficult to forecast how safe longer combination vehicles (LCVs) would be in operating environments other than the ones in which they have been allowed to operate to date. These multitrailer combinations currently operate at weights well above the 80,000-pound (36,288-kilogram) federal gross vehicle weight limit, primarily on low-volume rural roads in western states or on turnpikes in several eastern states. In those environments, their crash rates generally have been comparable to conventional tractor-semitrailer combinations, but many people question their safety on more congested roads in other parts of the country. LCVs inherently have stability and control limitations because of their length and number of trailers.

To understand the views of the many groups with an interest in TS&W limits, extensive outreach was conducted in this study. Outreach included public meetings; focus groups with various interested parties; workshops to review data and analytical methods used in the study; video conferences with state representatives; and requests for comments on study plans, working papers, and drafts of key parts of the report. These outreach activities confirmed the complexity and degree of concern surrounding many TS&W issues.

graphic of factors affecting federal truck size and weight law
Figure 1 - Factors affecting federal truck size and weight law.

Study Approach

The study was closely coordinated with the 1997 Federal Highway Cost Allocation Study to ensure that: (1) consistent assumptions were used in the two studies, (2) consistent methods were used to estimate infrastructure and other impacts of highway use by different vehicle classes, and (3) cost recovery and equitable user-fee issues could be addressed if they came up in the TS&W study or legislative proposals subsequent to completion of the study.

Based on this analysis, a number of factors that must be considered in the evaluation of potential changes in TS&W limits were identified. Those factors are shown in Figure 1.

State-of-the-art methods for assessing potential impacts of TS&W options were examined. Safety, productivity, infrastructure impacts (pavements, bridges, and geometrics), traffic congestion, environmental impacts (primarily air quality and noise), and impacts on railroads were considered the most important factors.

A major part of the study involved developing and testing analytical tools to estimate potential diversion of freight from one type of truck to another or between rail and truck if TS&W limits were changed. This study makes several significant improvements over previous studies by explicitly considering inventory and other logistics costs that shippers evaluate in making real-world transportation decisions and by analyzing, in detail, large numbers of specific moves rather than a few typical moves.

Like previous studies, this study analyzes several specific TS&W scenarios characterized by assumptions about the maximum weights and dimensions of vehicles that would be allowed to operate and about the networks upon which larger, heavier vehicles could travel. Many potential scenarios could be identified, but resource constraints limited the number of illustrative scenarios that could be analyzed. While most scenarios assume some increase in TS&W limits, two scenarios assume reductions in allowable weights or dimensions.

The safety analysis includes an extensive review of past safety studies and a synthesis of results that could be pulled from those studies. An important contribution of this study is the development of tools to evaluate stability and control properties of different vehicle configurations at different weights and dimensions. Differences in vehicle stability and control are perhaps the most important safety-related factors directly related to differences in vehicle weights and dimensions. Where crash rates and other direct evidence of the relative safety of certain vehicles are not available, the stability and control characteristics of the vehicle provide an indication of its relative safety compared to vehicles currently in widespread use.

diagram of illustrative vehicle configurations
Figure 2 - Illustrative vehicle configurations.

Illustrative Truck Size and Weight Scenarios

Five TS&W scenarios were developed for this study to illustrate the nature and relative magnitude of impacts on safety, productivity, infrastructure, the environment, traffic operations, and the railroads. Scenarios are characterized by specific vehicles that would likely operate under the scenarios, gross weight limits and lengths at which those vehicles would operate, and the network of highways upon which scenario vehicles would operate and the federal TS&W limits would apply. Figure 2 shows generic vehicle configurations that were analyzed in various scenarios. Assumptions for each of the illustrative scenarios are briefly described below.

Uniformity Scenario

This scenario assumes that grandfather provisions in current federal law would be removed and requires that states adopt federal weight limits on all highways in the National Network (NN), which was established in the 1982 Surface Transportation Assistance Act. States now exercising grandfather rights to allow heavier vehicles on the Interstate Highway System would have to roll those weights back to the current federal limits. They also would have to roll back any higher limits that they may now have on other NN highways. With an 80,000-pound weight limit, LCVs would be impractical for all but the lightest loads. A few states have weight limits below federal limits on non-interstate portions of the NN. Those states would be required to bring weight limits up to federal limits on those NN highways. Non-divisible load permits would continue. Off the NN, vehicles would continue to operate at current state-regulated weights.

graphic of states allowing various longer combination vehicles
Figure 3 - States (in darker shade) allowing various longer combination vehicles.

North American Trade Scenarios

The North American trade scenarios allow heavier gross vehicle weights on certain configurations by increasing allowable tridem-axle loads to be more consistent with tridem-axle loads in Canada and Mexico. Two alternative tridem-axle load limits are tested, one at 44,000 pounds (20,000 kilograms) and the second at 51,000 pounds (23,000 kilograms). This second limit would allow transportation of international containers loaded to the International Standards Organization (ISO) limit. Gross weights of six-axle tractor-semitrailers carrying those containers would be 97,000 pounds (44,000 kilograms). Other vehicles considered in this scenario are a four-axle single-unit truck weighing up to 71,000 pounds (32,000 kilograms) and an eight-axle twin-trailer combination weighing up to 131,000 pounds (almost 60,000 kilograms) with trailer lengths of 33 feet (10 meters). Because they corner as well as current tractor-semitrailers, the eight-axle twin-trailers would be allowed the same access. Eight-axle doubles are operated in some Canadian provinces and in states along the U.S.-Canadian border, but not in Mexico. Current grandfathered weight limits would stay in effect in these scenarios.

Longer Combination Vehicles Nationwide Scenario

Longer combination vehicles currently operate in 16 states west of the Mississippi River and on turnpikes in five states east of the Mississippi River. Figure 3 shows states that allow various types of LCVs. The Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) contains an "LCV freeze" that prevents expansion of LCVs into states that did not permit those vehicles before June 1, 1991.

The LCVs nationwide scenario assumes LCV operations on a nationwide network. Limited networks would be designated upon which LCVs could operate. Turnpike doubles, including the twin 53-foot (16.2-meter) trailer combination weighing 148,000 pounds (67,000 kilograms) that was tested in this scenario, and Rocky Mountain doubles, including the combination of one 53-foot trailer and one 28.5-foot (8.7-meter) trailer weighing 120,000 pounds (54,000 kilograms) used in the scenario, would not be allowed to leave the network because of their relatively poor maneuverability. They would have to use staging areas to assemble and disassemble, and travel off the network would be in single-trailer combinations. Triple-trailer combinations with three 28.5-foot trailers weighing up to 132,000 pounds (60,000 kilograms) and eight-axle twin-trailer combinations with two 33-foot trailers weighing up to 124,000 pounds (56,000 kilograms) would be allowed to travel off their networks to get from origins and to destinations because they can negotiate curves as well as current tractor-semitrailer combinations.

H.R. 551 Scenario

The Safe Highways and Infrastructure Preservation Act was first introduced in 1994 during the 103rd Session of Congress and again in 1997 as H.R. 551 during the 105th Session. The bill, which was under consideration at the time of the study, would federalize certain areas of truck regulations that are now state responsibilities. Specifically, H.R. 551 contains three provisions related to federal TS&W limits: (1) It would phase out trailers longer than 53 feet. (2) It would freeze state grandfather rights. (3) It would freeze weight limits (including divisible load permits) on non-interstate portions of the National Highway System.

Triples Nationwide Scenario

This scenario assumes operation of triple-trailer combinations across the country at the same weights and dimensions as are assumed under the LCVs nationwide scenario.

Table of Estimated Diversion for Selected Vehicle Configurations
Table 1 - Estimated Diversion for Selected Vehicle Configurations for Illustrative Truck Size and Weight Scenarios.

Illustrative Scenario Impacts

Table 1 shows estimates of the diversion of freight from existing trucks and from rail to selected vehicles for each of the scenarios. Total vehicle-miles of travel (VMT) do not equal the sum of VMTs for individual vehicle classes because not all vehicle classes are shown. Also, it should be pointed out that total national truck VMT for all scenarios is greater than current levels due to the overall growth in the national economy forecast over the study period.

The two illustrative scenarios involving some rollback of state TS&W limits show small increases in travel by five-axle tractor-semitrailer combinations and small increases in total heavy-truck VMT. The uniformity scenario would reduce travel by six-axle tractor-semitrailers and LCVs because those vehicles would not be able to travel at weights above 80,000 pounds on the NN. The H.R. 551 scenario has very small changes in VMT for these two vehicle classes.

The four scenarios allowing heavier vehicle weights all show large (greater than 70 percent) reductions in travel by five-axle tractor-semitrailers and very large increases in LCV travel. Total VMT estimated under the North American trade scenarios is about 10 percent less than total base-case VMT.

Most VMT that shifts from five-axle tractor-semitrailers diverts to eight-axle twin-trailer combinations rather than six-axle tractor-semitrailers in the North American trade scenarios because the twins are assumed to have wide access off the NN and have significantly greater cubic capacity and vehicle weight. In fact much of the diversion to the eight-axle twins is lower density traffic that takes advantage of the additional cubic capacity of the vehicle rather than the additional gross weight it can carry compared to the six-axle tractor-semitrailer.

Table of Estimated Impacts of Illustrative Truck Size and Weight Scenarios
Table 2 - Estimated Impacts of Illustrative Truck Size and Weight Scenarios (Percent Change from Base Case.)

Impacts of the various TS&W scenarios on infrastructure, shipper and rail costs, and the environment are all related to the traffic diversion estimates summarized above. Table 2 shows estimated changes from base-case levels for key impact areas. Bridge replacement costs change significantly under all scenarios, including those that would reduce certain vehicle weights and dimensions. The assumption in this study is that all bridges that would be stressed beyond the overstress criteria underlying the federal bridge formula ultimately would be replaced to accommodate vehicles allowed under the various scenarios. This is similar to assumptions in previous TS&W studies by DOT and TRB; however, based on comments by several states, it may overestimate bridge-related costs. In practice, depending on the degree of overstress, on the volume of vehicles expected to use the bridge, and on the type of bridge, states might postpone replacement for a number of years or perhaps be able to strengthen the bridge rather than replace it. Impacts of heavy trucks on fatigue and bridge deck deterioration are not estimated. An on-going study under the National Cooperative Highway Research Program is examining fatigue and deck deterioration issues in more detail.

While bridge costs are primarily a function of weight, geometric costs are strongly influenced by trailer length. In general, the longer the trailer, the greater the vehicle's offtracking, especially in multitrailer combinations. (Offtracking is a measure of the extent to which the rear axles of a vehicle follow a different path than the front axles when the vehicle is negotiating a turn. If there is significant offtracking, the vehicle cannot stay within its lane when going around a corner and may have to travel on the shoulder on a tight interchange ramp.) Some freeway interchanges and at-grade intersections would have to be modified to accommodate the offtracking of longer vehicles.

In scenarios analyzed for this study, turnpike doubles and Rocky Mountain doubles are assumed to be restricted to limited networks. Staging areas would be required to allow those vehicles to assemble and disassemble for travel off those networks. Some western states currently allow those vehicles to travel more widely than is assumed in the illustrative scenarios, but the vehicles operating in those states are shorter and lighter than the configurations examined in this study. The additional length would make the scenario vehicles less maneuverable than the vehicles in use today.

As in other TS&W studies by DOT and TRB, this study estimates that certain scenarios could produce significant reductions in shipping costs. Changes in shipping costs, shown in Table 2, are all smaller in percentage terms than changes in some other impacts; however, the base for these changes is much larger. Assumptions about allowable vehicle weights and dimensions and the extent of the network available for LCVs result in estimates of shipper cost savings that are higher than estimates in most previous studies. If lower weights, shorter lengths, and smaller networks were analyzed, shipper cost savings would be lower, but so too would be most of the other impacts.

The analysis of scenario impacts on rail revenues indicates that several scenarios could significantly reduce revenues available to cover railroad fixed costs, known as "contribution." Because contribution is closely linked to return on investment, contribution is an important measure of a railroad's ability to cover its fixed cost and sustain necessary ongoing investment. Industry-wide estimates showed that contribution could be reduced by more than 50 percent under the LCVs nationwide scenario and by lesser amounts under the North American trade and triples nationwide scenarios, which also allow nationwide operation of LCVs.

Safety impacts are not shown on this table because there are so many dimensions to the safety issue that no one adequately captures safety considerations surrounding the illustrative scenarios. The LCV configurations generally show poorer stability or control properties than the base tractor-semitrailer configuration. Short multitrailer combinations have poor lateral stability that can result in the rearmost trailers traveling outside their lane or, in the extreme case, rolling over if rapid steering maneuvers are required. In general, the shorter the trailer, the worse the lateral instability, although certain types of trailer connections can improve stability. Thus, while shorter trailers on triple-trailer combinations reduce offtracking, they also reduce lateral stability. Reducing the allowable weights and dimensions of scenario vehicles would improve stability and control; however, it would also reduce productivity for many segments of the trucking industry.

Conclusions

Significant productivity benefits are estimated for each illustrative scenario that allows heavier vehicle weights, but these benefits are derived primarily from the use of LCVs, even under the North American trade scenarios. Nationwide use of LCVs would entail significant infrastructure costs, adverse impacts on railroads, and potentially negative safety impacts. Furthermore, officials in many states that currently do not allow LCVs oppose policies that would relax restrictions on LCV use. In addition to concerns about infrastructure costs and safety risks, their opposition likely reflects apprehension about larger trucks by motorist and other interest groups in their states.

States differ markedly on their positions regarding changes in federal TS&W limits. Some states oppose changes in federal TS&W laws that would give states either the flexibility to allow higher gross weights or to allow LCVs. In general, they fear that if neighboring states allow LCVs, they will face irresistible pressure to also allow LCVs in order to keep their businesses competitive.

States that presently allow LCVs on their state highways generally favor removing the LCV freeze and liberalizing rules under which LCVs may operate. They argue that grandfathered operations in most states are based on laws in effect in 1956 and that highways have become safer since that time. They also maintain that LCVs have had good safety records in their jurisdictions, improve productivity, and can operate on their highway systems without staging areas or interchange improvements. They also point out that current grandfather laws often result in LCVs having to operate off the Interstate Highway System rather than on the safer interstate highways.

Other states would like increases in the gross weights allowed for six-axle tractor-semitrailers and single-unit trucks such as dump trucks, garbage trucks, and other specialized hauling vehicles. These states want additional truck productivity without the infrastructure costs and potential safety concerns associated with LCVs. No separate analysis was conducted in this study to estimate the effects of allowing only those shorter vehicles. In general, such vehicles would not be expected to cause additional pavement damage on interstate highways, nor would they increase the cost of improving roadway geometrics.

Bridge impacts would be mixed depending on the gross weights allowed. The heavier vehicles allowed under the North American trade scenario would require substantial bridge improvements. Heavier six-axle tractor-semitrailers, such as the 97,000-pound vehicle that would be allowed to operate under H.R. 1667 introduced in 1999, generally would exceed bridge formula limits and would cause stresses exceeding bridge design stresses.

While basic federal TS&W limits have not changed since 1982 with the exception of the LCV freeze, this does not mean that the status quo has been maintained. Since 1982, several states have been granted exceptions to federal gross weight or axle-weight limits in either authorizing or appropriating legislation, including four states that received such exemptions in the Transportation Equity Act for the 21st Century (TEA-21) in 1998. States are granting increasing numbers of oversize and overweight permits, especially for international containers. The cubic capacity of vehicles has also changed, primarily as a result of increasing trailer length. For example, at the time of DOT's last comprehensive report on TS&W policy issues in 1981, the standard trailer length was 45 feet (13.7 meters), and 48-foot (14.6-meter) trailers were becoming increasingly common. Now, 53-foot- (16.2-meter-) long semitrailers are becoming a standard for many carriers, and some states allow trailers up to 60 feet (18.3 meters) in length. Average operating weights of tractor-semitrailers have actually gone down slightly in recent years with decreases in cargo density and pressures to provide smaller, more frequent deliveries to support just-in-time and other advanced logistics operations.

Several implications of these ad hoc trends are occurring while basic federal TS&W limits remain unchanged. With the increasing weights being allowed under permit, pavements and bridges will deteriorate more quickly. Increasing trailer lengths probably have not had as significant an effect because carriers are operating those vehicles with the rear axles pushed forward so that their offtracking is not significantly worse than 48-foot trailers. However, as trailer lengths have moved beyond 53 feet in some states, geometric deficiencies have increased because there is a limit to how far forward the rear axles can be pushed to minimize offtracking. The sum of these ad hoc changes at the state level has been to create an ever more diverse patchwork of TS&W limits nationwide. Increasing trade with Mexico and Canada, which have higher allowable gross weight and axle weight limits than the United States, will cause even greater pressure to increase weight limits in this country, especially in major trade corridors.

Cost recovery is an issue that several states mentioned in comments to the docket, and it is an issue for the federal government as well. Most increases in TS&W limits would require some infrastructure improvements. Even if more incremental changes in TS&W limits were implemented than those included in the illustrative scenarios, bridge, geometric, and pavement costs could increase. Some states capture a large share of the additional infrastructure costs associated with the operation of oversize and overweight vehicles through permit fees, but other states charge fees that cover little more than the cost to administer the permit program. On the federal level, there is no mechanism for capturing the added cost of larger, heavier trucks through user taxes. Weaknesses of the current federal user fee structure to reflect the cost responsibility of different vehicle classes were discussed in detail in the 1997 Federal Highway Cost Allocation Study.

TRB has a study underway of federal TS&W regulations. That study, called for in TEA-21, will consider whether changes in federal TS&W limits are advisable and will evaluate how changes might affect the economy, environment, safety, and services to communities.

DOT will continue to improve this analytical framework during the next several years. Comments submitted to the docket provided valuable recommendations for additional research in several areas. In May 2000, the Federal Highway Administration sponsored a nationwide TS&W policy workshop to discuss specific improvements that can be made in the data and the analytical methods used to assess the impacts of TS&W policy options. The workshop was a forum for stakeholders to provide their perspectives on future directions for federal TS&W policy.

The analytical framework developed for this study is flexible, and many assumptions can be varied to assess specific proposals. While the illustrative scenarios analyzed in this study covered most basic TS&W alternatives, many variations are possible. An option might be identified that could improve shipper and carrier productivity, improve safety, have acceptable infrastructure costs, and have little effect on railroads or other modes. Identifying such an option would require close coordination with states, shippers, carriers, and other industry groups. If consensus could be developed that the benefits clearly outweighed the potential cost, it might be possible to rationalize national TS&W policy; reduce or eliminate the need for the kinds of state exemptions to federal TS&W laws that recently have been enacted; and improve safety, productivity, and international competitiveness.

James W. March is the team leader of the Industry and Economic Analysis Team in FHWA's Office of Transportation Policy Studies. In addition to overseeing completion of the Comprehensive Truck Size and Weight Study, he managed work on the 1997 Federal Highway Cost Allocation Study. March joined the Federal Highway Administration in 1969. His career has included a variety of assignments in the Office of Policy and its predecessor organizations; these assignments include chief of the Economic and Demographic Forecasting Branch and chief of the Systems Analysis Branch. March has a bachelor's degree in economics from the University of Virginia, and he completed the FHWA graduate study program in civil engineering at Virginia Polytechnic Institute and State University.

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