TRB Committee on Motor Vehicle Size and Weight - A1BO4

Originally published: December 1999


Commercial motor carrier freight transportation will continue to play a vital role in the U.S. economy.  In 1992, for-hire and private trucking accounted for 65 percent of the total contribution that transportation related services made to the U.S. GDP, and 3.3 percent of the total U.S. GDP[1].  That year, 6 million medium and heavy commercial trucks traveled a total of 152.5 billion miles[2], (nearly 7 percent of all vehicle miles of traveled that year) hauling freight in this country.  Travel is growing at a 2.5% annual rate, with combination-unit commercial truck travel growing at an even higher 3.5 percent rate.  Trucks now routinely approach 40 percent of the traffic mix on certain segments of interstate highways, at various times of the day, with overall traffic densities frequently approaching or exceeding maximum free-flow capacity limits.  The truck portion of the traffic mix will likely continue to increase.  Simultaneously, truck accidents and fatalities are rising, as is public concern about this trend.  Against this backdrop, there will be continuing strong political debate and economic pressure to increase maximum allowable truck size and weight limits as a way of handling both the need for productivity improvements, and this large and growing amount of travel.  Accommodating this seemingly straightforward goal, however, will be anything but simple.  Multiple, potentially conflicting, public policy issues are involved.

In recent years there has been a significant increase and improvement in the body of objective information upon which any decision to address this issue might be based, as more and higher quality technical and public policy research has become available.  For example, the effects of, and costs attributable to individual axle weights and arrangements on pavements -- and overall weight and vehicle configuration on bridges, has been more clearly identified.  Similarly, the interaction between vehicle configurational layout, road geometries, and traffic operational effects has been better defined.  Also, methodologies for assessing the effects of various vehicle designs, configurations, axle arrangements, weight, and cargo loadings on vehicle dynamic handling and stability performance -- and in turn, safety -- have been developed.  This creates the possibility that objective performance-based evaluation methodologies and acceptability criteria could be developed relative to these issues -- which in turn, leads to the opportunity to develop responsible, reasonable, balanced tradeoffs.

In the public policy arena, new information is available for assessing not only the direct highway infrastructure-related costs occasioned by the use of larger, heavier vehicles, but also the degree to which federal truck user fees cover those costs[3].  Also, the range of potential economic consequences on other freight transportation modes - principally railroads, which might occur if freight traffic shifted from railroads to trucks, can be assessed.  Safety consequences and public sentiment relative to sharing the road with larger, heavier trucks have also been gauged.  These results create the possibility that reasonable, balanced accommodations might be possible relative to these issues as well.

To achieve this balance and accommodation, some type of objective, rationalized approach to addressing truck size and weight policy would have to be developed.  This would be significant departure from the present situation.   As presently conceived, U.S. truck size and weight limitations are a mishmash of federal, state, and local requirements, whose origin and development were evolutionary and jurisdictionally fragmented.  Lack of any rationalized approach to establishing these standards has resulted in significant differences among jurisdictions - principally on the non-interstate portion of the nation’s roadway system.  Federal requirements generally tend to have an overall limiting effect, since most vehicles, at one time or another, operate on the Interstate and/or National Highway System.  These differences limit a given motor carrier’s operational flexibility to an increasing degree, depending on whether those operations are local, regional and/or national in scope.  Thus, if rationalization is to occur, it appears it would have to be done at the federal level, within a framework that allows significant latitude, flexibility and decision-making at the state and local levels.

The results of all the research currently available do not point to easily achieved consensus on wholesale use of significantly larger vehicles on a national basis.  Nevertheless, the results can be inferred to indicate there may be opportunities, on a national basis, for using better designed and performing vehicles which were somewhat larger than those in present use, while simultaneously allowing more constrained and controlled use of better performing vehicles, which were larger yet, in locations and regions where their restricted use would not pose inordinate safety risks nor create competitive imbalances.  All of these possibilities carry with them the presumption that an effective safety control and sanctioning system could be put in place, and that adequate cost recovery mechanisms would be established to capture the appropriate level of costs occasioned by the use of these vehicles.

Going forward, the debate will center on whether, and under what conditions - if any, it would be good public policy to allow larger and heavier trucks to operate.  At different times, the question will likely be framed in terms of allowing incremental increases locally (wholly within a state) or regionally (within a limited number of adjoining states), while in other cases, national uniformity at increased levels will be sought to allow national network or corridor type operations.  Nevertheless, because of the overall limiting nature of federal requirements, the debate will be played out principally on the national stage.  Achieving consensus will be a significant challenge, but continuing to attack the problem in piecemeal fashion does not appear to be in any of the involved parties’ best long-term interests.  Success can, and will only be achieved, when and if win-win solutions are found that balances all the interests’ concerns, and which address:

  • Safety, including the enforceability and practical ease of implementing any safety regulations or other countermeasures on a uniform and rigorous basis across the jurisdictions where larger, heavier vehicles might be allowed;
  • Incrementally higher infrastructure costs attributable to the use larger, heavier vehicles (pavements, bridges, and geometric design) and how these costs would be recouped;
  • Sorting out federal, state, and local government roles in regulating traffic and equipment, as well as interstate and international commerce;
  • Flexibility to accommodate differences in transportation requirements across regions and commodities;
  • Impacts on all freight shippers, other transportation modes, and intermodal movements;
  • Environmental and other quality of life issues, and;
  • Effects on the efficiency and safety of automobile travel.

One way of accomplishing all these goals might be to broadly differentiate between the large population of current "typical" vehicles (i.e., 2 and 3-axle single-unit trucks<54,000 lbs, and combination-units<80,000 lbs.), and the much smaller specialized group of high-productivity, larger vehicles.  This latter group presents unique concerns that warrant dealing with them separately, and more rigorously.  Past studies have pointed to a nationally uniform, permissive, state-based special permit program as a basis for discriminately dealing with all the concerns that need to be addressed relative to this latter group of vehicles.  Such a program, if properly structured, could be objective, systematic, rigorous, and credible.  This basic approach/concept has been endorsed in TRB Study 225 and the trucking industry and could be the basis of a collaborative federal/state initiative.

Since safety concerns have been the principal stumbling block to movement on this issue, it seems appropriate to focusing first on structuring an approach to addressing this concern.  Focusing then on the safety issue, it appears that successful U.S. expansion of the use of larger vehicles will only be possible if some nationally agreed-upon set of standards and conditions of use can be established and rigorously followed.  These standards would ensure that the basic designs of the vehicles were optimized for safety, and that their use was restricted to only those locations where it was appropriate. 

Any program to accomplish this should have at its core state, and in some cases local level self-determination of appropriate truck sizes and weights within that jurisdiction.  In the case of larger/heavier trucks, (i.e., single-units>54,000 lbs. and combination-units>80,000 lbs.) it is unlikely national consensus could be reached on the appropriateness of allowing all the sanctioned or "approved" vehicles or vehicle configurations which might be deemed acceptable, to operate everywhere.  What might be possible, however, would be an array or "catalog list" of potentially "safety acceptable", "blueprint" vehicles/configurations, from which jurisdictions could "choose" if they decided to allow larger/heavier vehicles in their jurisdiction. 

Depending on the capabilities of each jurisdiction's infrastructure, and prevailing traffic conditions, some vehicles and configurations could be acceptable, under some circumstances.   The process of determining acceptable "blueprint" vehicles could effectively be sanctioned and cataloged through organizations such as SAE, eliminating concern that this process would be too complicated.  AASHTO could be enlisted to develop a methodology for assessing the ability of a given roadway segment, coupled with its typical traffic volumes, to handle the array of "blueprint" vehicles that would available.

National concerns would be addressed under such an approach if credible assessment methodologies and acceptability criteria were established in four areas:

  • Adequacy of user fees to recoup user-occasioned federal-aid system costs attributable to the larger vehicles, to provide funding for  adequate staffs at the state level to administer adequate special permit programs and, to support vehicle and route certification efforts;


  • Infrastructure preservation criteria – primarily pavement and bridge wear;
  • Safety assurance -- primarily ensuring that truck dynamics/stability properties are as good as or better than current vehicles, ensuring that excessive negative traffic operations interactions with cars do not develop, and tracking carrier performance and adherence to program requirements with an appropriate data collection and reporting system, and:
  • Funding and implementation of rigorous enforcement programs with effective sanctions, fines, and other mechanisms which would all but eliminate incentives to violate the agreed-upon standards.

The "acceptability" of a given vehicle or vehicle configuration, as well as programs permitting their use, would be gauged relative to their ability to accomplish these objectives.  Programs to ensure these objectives are met need would need to be substantive, verifiable, performance-based, and uniformly implemented.  Uniformity is the key.  Without it, jurisdictions attempting to maintain a responsible level of stringency in their programs could be viewed as being unnecessarily and overly restrictive, and would find themselves in an untenable political disadvantage, if neighboring states had less restrictive requirements.  History has shown that the net effect would be a "lowest common denominator" set of requirements that many would view as being inadequate.

Such a system could provide enough flexibility to credibly allow use in locations and regions where risk would be minimal and controlled, while ensuring that in higher risk situations use would either be more constrained or not allowed.  Also, since national use would likely be constrained, modal competitive imbalance issues would likely be minimized, if not eliminated as a cause of concern.   Establishing such a system would be a formidable, but not impossible task, requiring the cooperation and input of a large number of potentially affected groups.  Time will tell if a balanced, comprehensive program like this, or some other, can be agreed upon.


[1] “Transportation Statistics Annual Report: Long Distance Travel and Freight”, USDOT/BTS, 1998 

[2]  “Highway Statistics 1992", USDOT/FHWA

[3]  see "1997 Federal Highway Cost Allocation Study", for a discussion of cost responsibilities and federal user charges for vehicles weighing more than 80,000 lbs.