Summary Report

Introduction

This report presents results of a comprehensive examination of issues surrounding current Federal truck size

and weight (TS&W) limits and potential impacts of changes to those limits. This is the Department of

Transportation’s first comprehensive study of Federal TS&W limits since 1981. However, the

Department, the Transportation Research Board (TRB), and others have conducted a number of studies

over the past 20 years of various aspects of Federal TS&W regulation. Those studies have highlighted the

diversity of opinions among States, different segments of the trucking industry, and various other interested

groups. 

While these studies have generally included options to either increase or decrease Federal TS&W limits,

attention has focused on options to improve productivity through various increases in TS&W limits. This

follows from ad hoc changes that have been occurring in truck sizes and weights such as the gradual

increases in trailer lengths over the years and the increasing numbers of overweight permits being issued by

States.

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 to 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. Only general estimates of these costs can be made since it is impossible

to predict the extent to which States would allow larger and heavier vehicles to operate if no uniform

nationwide criteria were in place. 

 

Safety has been one of the issues of greatest concern in previous TS&W studies, yet it is difficult to

quantify many safety impacts. Motorists are keenly aware of the growing volume of trucks on the road,

and many express discomfort when driving in traffic with many large trucks. Particularly difficult to

estimate is how safe longer combination vehicles (LCVs) would be in operating environments other than the

ones in which they have been allowed to operate in the past. These multitrailer combinations currently

operate at weights well above the 80,000-pound 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 question

their safety on more congested roads in other parts of the country. LCV’s have inherent stability and

control limitations because of their length and number of trailers. Short trailers tend to decrease vehicle

stability and long trailers decrease vehicle control

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, regional focus groups with various interested

parties, workshops to review data and analytical methods used in the study, requests for comments on study

plans, working papers, and drafts of key parts of the report, and video conferences with State

representatives. These outreach activities confirmed the complexity and degree of concern surrounding

many TS&W issues.
2

Various segments of the trucking industry view TS&W regulation differently, based on their assessment of

how it would affect their competitive and financial position. Not all segments of the industry believe they

would benefit from increased size and weight limits. States also disagree on the appropriate Federal TS&W

policy. Some States want the flexibility to set TS&W limits on all their highways including those on the

Interstate System. Other States prefer stronger Federal control over TS&W limits to minimize pressures for

increased weights and dimensions. 

Background

1956-1975

The Federal Government did not begin regulating TS&W limits until 1956 when maximum vehicle weight

and width limits were imposed on vehicles operating on the new Interstate Highway System. States

historically had regulated the weights and dimensions of vehicles operating on State highways, but Congress

believed that the large Federal investment in the Interstate System required more direct Federal controls on

the weights of vehicles using the Interstate System. A maximum gross weight limit of 73,280 pounds was

established along with maximum weights of 18,000 pounds on single axles and 32,000 pounds on tandem

axles. Maximum vehicle width was set at 96 inches, but length and height limits were left to State

regulation. States having greater weight or width limits in place on July 1, 1956 when Federal limits went

into effect were allowed to retain those limits under a grandfather clause.

The Congress increased allowable gross weight and axle weight limits in1975, in part to provide additional

cargo-carrying capacity for motor carriers faced with large fuel cost increases at the time. The gross vehicle

weight limit on Interstate Highways was increased to 80,000 pounds and single and tandem-axle load limits

were increased to 20,000 pounds and 34,000 pounds respectively. As in the 1956 Act, these limits were

permissive and States could adopt lower limits if they chose. In the same legislation Congress required that

each State annually certify that it was enforcing all State size and weight laws on all Federal-aid highways

and provided for highway funding sanctions if States were found not to be adequately enforcing their

TS&W laws.

1982-1991

Not all States immediately adopted the 80,000-pound weight limit. Motor carriers traveling through a State

that retained the 73,280-pound limit had to restrict their loads to that weight even though most States had

adopted the 80,000-pound weight limit. Carriers were most concerned about a small group of “barrier

States” along the Mississippi River that retained the 73,280- pound limit and effectively limited much of the

East-West traffic crossing the Mississippi River to the lower weights in those States. Carriers were also

concerned that over the years State length limits and regulations on the use of short twin-trailer

combinations were creating inefficiencies. A study called for in the Surface Transportation Assistance Act

(STAA) of 1978 addressed these and other issues. Based in part on results of that study, Congress, in the

STAA of 1982, required States to adopt the Federal weight limits on Interstate Highways and also required

them to allow vehicles with certain minimum dimensions on a National Network (NN) for STAA vehicles to

be designated by the Secretary of Transportation in consultation with the States. In particular, the STAA of

1982 required States to allow tractor-semitrailer combinations with 48-foot long semitrailers and twin- trailer

combinations with trailers of 28 feet to operate on the NN.
3

Current Federal law includes the following limits:

• 20,000 pounds for single axles on the Interstate System;

• 34,000 pounds for tandem axles on the Interstate System;

• Application of the Federal Bridge Formula for other axle groups up to the maximum of

80,000 pounds gross vehicle weight on the Interstate System;

• 102 inches for vehicle width on the National Network (NN) for STAA vehicles;

• 48-foot (minimum) or longer if grandfathered, for semitrailers in a semitrailer

combination on the NN; and

• 28-foot (minimum) for trailers in a twin-trailer combination on the NN.

Figure 1. Current Federal Truck Size and Weight Limits

Figure 1 summarizes current Federal TS&W limits. Implementation of these provisions was difficult,

especially in States that previously had not allowed twin-trailer combinations. Requiring States to allow

certain vehicle weights and dimensions on the Interstate System established a much stronger Federal role in

the area of TS&W regulation than the Federal Government had assumed before. In addition to requiring

that the STAA vehicles be allowed to operate on the NN, those vehicles were to be granted reasonable

access off the NN to terminals and to facilities for food, fuel, rest, and repairs. Defining the extent of the

NN and what “reasonable access” meant was a controversial process.

In addition to TS&W provisions, the STAA of 1982 also increased the Federal fuel tax by 5 cents per

gallon and increased other Federal user charges on heavy trucks, based in part on work conducted for the

1982 Federal Highway Cost Allocation Study. The cost allocation study had found that Federal user taxes

being paid by heavy trucks were not covering pavement, bridge, and other infrastructure costs attributable to

those vehicles. Changes in user fees enacted in the STAA of 1982 resulted in heavy trucks paying a larger

share of their highway cost responsibility. 

1991 to Present

The most significant legislative action related to Federal TS&W limits since 1982 was the freeze on LCV

operations imposed in the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) (P.L. 102-

240) and extended in the Transportation Equity Act for the 21st Century (TEA-21) (P.L. 105-85). Several

studies in the 1980s by the Department of Transportation and the Transportation Research Board (TRB)

had examined TS&W options involving LCVs. As noted above, such vehicles have operated in many

western States and on some eastern turnpikes for a number of years, but the possibility that Federal TS&W

limits might be changed to allow those vehicles to operate more widely was, and continues to be,
4

widely debated. The “LCV freeze” enacted in the ISTEA prohibited States from allowing any expansion of

LCV operations either in terms of routes upon which they may operate or the vehicle weights or dimensions

that may be allowed.

Over the years special exemptions to Federal weight limits have been enacted for individual States,

sometimes applying only to the transportation of specific commodities that are important to the State

economy. These special exemptions along with the grandfather rights allowing States to operate vehicles

exceeding Federal weight limits have created what many have characterized as a patchwork of Federal

TS&W limits on Interstate Highways that overlays an even more diverse set of State TS&W limits off the

Interstate System.

Since 1982, States, various segments of the trucking industry, shippers and other groups have proposed

changes to Federal TS&W limits. This Comprehensive Truck Size and Weight Study has developed a

framework to analyze a broad range of potential options and has used that framework to analyze several

types of changes that have been recommended by others. This information and the analytical tools

developed for the study provide a basis for assessing the various potential benefits and costs of alternative

TS&W policy options.

Study Approach

Review Process

This study used a variety of methods to develop information concerning potential impacts of TS&W

options. In addition to the extensive outreach process described above, an internal review process involving

all interested elements within the Department was instituted to assure that the full range of perspectives was

considered in the study. In particular, study oversight and direction was provided by a Departmental Policy

Oversight Group (POG), comprised of senior policy officials from the Office of the Secretary, the Federal

Highway Administration, the Federal Railroad Administration, the National Highway Traffic Safety

Administration, and the Maritime Administration. In addition to the POG, a Multimodal Advisory Group

(MAG) was established to ensure that major technical decisions shaping the study would be made on an

intermodal basis with consideration to potential effects that changes in TS&W limits might have on the

Nation’s total freight transportation system. Because the rail system is both a necessary and important

element of the Nation’s freight transportation system, the Department considered it critical to assess

potential effects on the rail industry that might be brought about by the introduction of larger, heavier

trucks.

The study was closely coordinated with the 1997 Federal Highway Cost Allocation study to assure 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. 

The purpose of the Department’s Comprehensive TS&W Study was to develop an information base and set

of analytical tools upon which to evaluate alternative TS&W options rather than to recommend TS&W

policy changes. To guide decisions concerning TS&W policy and other freight issues the
5

The Department of Transportation established eight principles to guide freight transport policy:

• Ensure a safe transportation system;

• Use advances in transportation technology to promote transportation efficiency and safety;

• Promote economic growth by removing unwise or unnecessary regulation and through the efficient

pricing of publicly financed transportation infrastructure;

• Protect the environment and conserve energy;

• Provide funding and a planning framework that establishes priorities for allocation of Federal

resources to cost-effective infrastructure investments that support broad National goals;

• Effectively meet our defense and emergency transportation requirements;

• Facilitate international trade and commerce; and

• Promote effective and equitable joint utilization of transportation infrastructure for freight and

passenger service.

Figure 2. National Freight Transportation Policy Statement (January 1997)

Department developed a National Freight Policy in 1997 that contains eight principles that it would use in

evaluating freight-related issues. Those guiding principles are summarized in Figure 2.

An important first step in this study was to review previous studies that had been conducted by the

Department, TRB, and others concerning TS&W and related truck safety issues. While safety was 

perhaps the most controversial issue in the study, a comprehensive analysis of truck safety issues was

beyond the scope of the TS&W study. Only those safety issues directly related to truck weights and

dimensions were considered in depth in this study. While broader truck safety concerns are quite important,

the scope of this effort was focused on incremental effects of possible changes in TS&W limits on truck

safety. 

Case Studies, Focus Groups

In addition to the literature review, a series of case studies was conducted to examine different aspects of

truck transportation in detail, including competition and cooperation between trucking and other modes of

freight transportation, especially rail. Specific questions were asked concerning the likely response by

different types of carriers and shippers to various changes in Federal TS&W laws. Focus group meetings

with auto and truck drivers in different parts of the country also were conducted to understand more clearly

the perceptions of drivers in different settings, including drivers who have been exposed to LCVs and those

who have not.
6

TS&W

Law

Enhances

Safety Optimize

System

Promote

Innovation

Effect

Uniformity

Facilitate

Trade

Protect

Infrastructure

Enhance

Logistics

Ensure

Compliance

Streamline

Procedures

Reduce

Expenditures

Enhance

Competitiveness

Protect

Environment

Conserve

Energy

Recover

Infrastructure Costs 

Reduce

Nation’s

Freight Cost

Promote

Flexibility

Figure 3. Factors Affecting Federal Truck Size and Weight Law

Impact Analyses

Through this reconnaissance process a number of factors were identified that must be considered when

assessing TS&W policy options. Those factors are summarized in Figure 3.

An important part of the study was to examine the state-of-the-art in assessing various impacts of TS&W

options. Impacts considered most important include safety, productivity, infrastructure impacts (pavements,

bridges, and geometrics), traffic congestion, environmental impacts (primarily air quality and noise), and

impacts on railroads. The most important factor affecting the magnitude of most impacts is the amount of

traffic that would switch to new truck configurations from existing trucks or from rail as the result of

changes in Federal TS&W limits. A major part of the study involved developing and testing analytical tools

to estimate this diversion. 

Previous TS&W studies have estimated diversion based primarily on differences in transportation costs

between moving goods in vehicles operating under current Federal TS&W laws and moving goods in

vehicles operating under either lower or higher limits. This study makes a significant improvement in the

diversion analysis by explicitly considering inventory and other logistics costs that shippers evaluate in

making real-world transportation decisions. 

Another major improvement in the diversion analysis is the use of disaggregate data on observed

movements from origin to destination over real transportation networks. Previous studies used aggregate

data characterized by region of the country and trip length distributions rather than actual shipment data.

Like previous studies, this study analyzes several specific TS&W scenarios characterized by assumptions 
7

about the maximum weights and dimensions of vehicles that would be allowed to operate and the networks

upon which larger, heavier vehicles could travel. Many different potential scenarios were considered, but

the detail at which network and access issues for each scenario were analyzed limited the number of

scenarios that could be included in the study. With the limited number of scenarios that could presented in

the report, scenarios were selected that showed the upper range of impacts that might be expected with

various types of TS&W changes. While most scenarios assume some increase in TS&W limits, two

scenarios assume reductions in allowable weights or dimensions.

Vehicle Characteristics

Many alternative vehicle configurations might be possible for a given set of TS&W limits. For analytical

purposes each scenario specified one or more “scenario vehicles” into which traffic from existing trucks or

from rail potentially could divert. Diversion estimates were based on truck traffic forecast for the year

2000, but it was assumed that all fleet changes and changes in shipper behavior, which in practice would

occur over many years following a change in TS&W limits, would take place by 2000. The alternative

would have been to try to estimate how long it would take for a new equilibrium to be achieved, and how

various shippers and carriers would respond while this new equilibrium was being established. As in

previous TS&W studies this was deemed to be too speculative so the assumption for analytical purposes

was that a final equilibrium would be achieved instantly.

 

While a limited number of “scenario vehicles” were used in the diversion analysis, those vehicles represent a

much larger array of vehicles that are in actual use. The highway cost allocation study analyzed 15 different

truck types, based on the number of trailing units and the number of axles. Impact analyses for this study

used those same vehicles classes. Figure 4 shows diagrams of many of those vehicle configurations. 

Table 1 shows the number of vehicles and the vehicle miles of travel for each of the 15 truck classes

included in the study. Estimates are shown for 1994 based on actual data and forecasts for 2000. The fiveaxle tractor-semitrailer accounts for 43 percent of all trucks and nearly two-thirds of all truck VMT. Threeaxle single units trucks are the next largest truck class with almost 25 percent of the vehicles and 8 percent

of the VMT. Longer combination vehicles, which include double-trailer combinations with 7 or more axles

and triples, currently account for less than 1 percent of all trucks and just over 1 percent of all truck VMT. 

Tables 2 and 3 show State truck weight and length limits that were in effect in 1994, the base year for this

analysis. Many exceptions to these limits exist for locally important commodities. Several important points

regarding State weight limits can be seen in Table 2. First, several States have higher weight limits off the

Interstate System than Federal law allows on the Interstate System. While some States have both higher

gross weights and higher axle weight limits, other States have the same gross weight limits, but different axle

weight limits or vice versa. 

Overlying the gross vehicle weight and axle weight limits on Interstate and other highways are systems of

overweight permits that are granted by each State. These permits are essential to allow non-divisible

loads to be transported, and often come with strict conditions under which the moves can be made.
8

Single Unit Trucks

Conventional Combination Vehicles

5-Axle Tractor Semi-Trailer 6Axle Tractor Semi-Trailer

STAA or “Western” Double

Longer Combination Vehicles (LCVs)

Rocky Mountain Double Turnpike Double

8-Axle B-Train Double Trailer Combination

Triple Trailer Combination

Figure 4. Illustrative Vehicle Configurations

 
9

Table 2 shows the weights at which “routine” overweight permits generally are issued. Fees are

charged for these permits which in some cases are intended to reflect the additional infrastructure costs

associated with the moves, but which in other cases only cover administrative costs of issuing the permits. 

Permits may be issued for moves at greater weights, but those moves often would require special equipment

and special routing.

Table 3 shows State length limits for semitrailers on Interstate and other State highways. In contrast to

State weight limits, where State length limits for semitrailers operating on Interstate and other highways

differ, the length limit on Interstate highways is typically longer, reflecting the better geometrics of Interstate

Highways. Some States do not regulate semitrailer lengths off the Interstate System, but have overall

vehicle length limits instead. Also of note is the fact that many States have maximum allowable distances

from the kingpin of the semitrailer to the rear axle. This controls vehicle off-tracking. Offtracking is

discussed in greater detail later in this report.

Vehicle Class

Number of Vehicles Vehicle Miles Traveled

(in millions)

1994 2000

Percent

Share of

Truck Fleet

1994 2000

Percent

Share of

Truck Fleet

3-axle single unit truck 594,197 693,130 24.9 8,322 9,707 7.6

4-axle or more single unit truck 106,162 123,838 4.4 2,480 2,893 2.2

3-axle tractor-semitrailer 101,217 118,069 4.2 2,733 3,188 2.5

4-axle tractor-semitrailer 227,306 265,152 9.5 9,311 10,861 8.5

5-axle tractor-semitrailer 1,027,760 1,198,880 43.0 71,920 83,895 65.4

6-axle tractor-semitrailer 95,740 111,681 4.0 5,186 6,049 4.7

7-axle tractor semitrailer 8,972 10,466 0.3 468 546 0.4

3- or 4- axle truck trailer 87,384 101,934 3.6 1,098 1,280 1.0

5-axle truck-trailer 51,933 60,579 2.2 1,590 1,855 1.4

6-axle or more truck-trailer 11,635 13,572 0.5 432 503 0.4

5-axle double 51,710 60,319 2.2 4,512 5,263 4.1

6-axle double 7,609 8,876 0.3 627 731 0.6

7-axle double 7,887 9,201 0.3 542 632 0.5

8-axle or more double 9,319 10,871 0.4 650 759 0.6

Triples 1,203 1,404 0.0 108 126 0.1

Table 1. Existing Truck Fleet and Vehicle Miles of Travel, 1994 and 2000 Projections
10

State

Gross Vehicle Single Axle Tandem Axle Federal Bridge “Routine” Permit

Interstate Other

Hwys.

Interstate Other

Hwys.

Interstate Other

Hwys.

Interstate Other

Hwys.

Gross

Vehicle

Weight

Single

Axle

Tandem

Axle

Alabama 80 84 20 20 34 40 Yes No-WT 110/150 22 44

Alaska -- 90(2) -- 20 -- 38 --- Yes 88.6(2)/150 30 50

Arizona 80 80 20 20 34 34 Yes No-WT 106.5(3)/250 28 46

Arkansas 80 80 20 20 34 34 Yes Yes 102/134 20 40

California 80 80 20 20 34 34 Yes-mod Yes-mod 119.8(4)/(5) 30 60

Colorado 80 85 20 20 36 40 Yes No 127/164 27 50

Connecticut 80 80 22.4 22.4 36 36 Yes Yes 120/160 22.4 NS

Delaware 80 80 20 20 34 40 Yes No-WT 120/120 20 40

D.C. 80 80 22 22 38 38 Yes-mod Yes-mod 155-248 31 62

Florida 80 80 22 22 44 44 Yes (6) No-WT 112/172 27.5 55

Georgia 80 80 20.34 20.34 34(7) 37.34 Yes Yes(6) 100/175 23 46

Hawaii 80.8 88 22.5 22.5 34 34 Yes No Case-by-case above normal limits

Idaho 80 105.5 20 20 34 34 Yes Yes Case-by-case above normal limits

Illinois 80 80(8) 20 20(9) 34 34(9) Yes Yes(9) 100/120 20 48

Indiana (10) 80 80 20 20 34 34 Yes Yes 108/120 28 48

Iowa 80 80 20 20 34 34 Yes Yes 100/160 20 40

Kansas 80 85.5 20 20 34 34 Yes Yes 95/120 22 45

Kentucky 80 80(11) 20 20 34 34 Yes Yes 96/140 24 48

Louisiana 80(12) 80(12) 20 22 34 37 Yes No 108/120 24 48

Maine 80 80(13) 20(14) 22.4 34 38 Yes-mod No 130/167 25 50

Maryland 80 80 20(15) 20(15) 34(15) 34(15) Yes Yes 110/110 30 60

Massachusetts 80 80 22.4 22.4 36 36 Yes Yes 99/130 NS NS

Michigan (16) 80 80 20 20 34 34 Yes Yes 80/164 13 26

Minnesota 80 80(17) 20 18 34 34 Yes Yes-mod 92/144 20 40

Mississippi 80 80 20 20 34 34 Yes Yes 113/190 24 48

Missouri 80 80 (18) 20 20(18) 34 34(18) Yes Yes(18) 92/120 20 40

Table 2. General State Weight Limits
State

Gross Vehicle Single Axle Tandem Axle Federal Bridge

Formula “B”

“Routine” Permit

Interstate Other

Hwys.

Interstate Other

Hwys.

Interstate Other

Hwys.

Interstate Other

Hwys.

Gross

Vehicle

Weight

Single

Axle

Tandem

Axle

Montana 80 80 20 20 34 34 Yes Yes 105.5/126 20 48

Nebraska 80 95 20 20 34 34 Yes Yes 99/110 20 40

Nevada 80 129(19) 20 20 34 34 Yes Yes 110(20)/(21) 28 50.4

New 80 80 20(15) 22.4 34(15) 36 Yes No 130/150 25 50

New Jersey 80 80 22.4 22.4 34 34 Yes No 100(22)/150( 25(22) 40(22)

New Mexico 86.4 86.4 21.6 21.6 34.32 34.32 Yes-mod Yes-mod 104(23)/120 26 46

New York 80 80 20(24) 22.4 34(24) 36 Yes(24) Yes(24) 100/150 25 42.5

North 80 80 20 20 38 38 Yes-mod Yes-mod 94.5/122 25 50

North Dakota 80 105.5 20 20 34 34 Yes Yes 103/136 20 45

Ohio 80 80 20 20 34 34 Yes No 120/120 29 46

Oklahoma 80 90 20 20 34 34 Yes Yes 95/140 20 40

Oregon 80 80 20 20 34 34 Yes/mod Yes-mod 90/105.5 21.5 43

Pennsylvania 80 80 20(25) 20(25) 34(25) 34(25) Yes(25) Yes(25) 116/136 27 52

Rhode Island 80 80 22.4 22.4 36 36 Yes-mod Yes-mod 104.8/(21) 22.4 44.8

South 80 80 20 22 34(26) 39.6 Yes(26) No 90/120 20 40

South Dakota 80 129(19) 20 20 34 34 Yes Yes 116(27)/(21) 31 52

Tennessee 80 80 20 20 34 34 Yes Yes 100/160 20 40

Texas 80 80 20 20 34 34 Yes-mod Yes-mod 106.1(28)/20 25 48.125

Utah 80 80 20 20 34 34 Yes Yes 100/123.5 20 40

Vermont 80 80 20 22.4 34 36 Yes Yes 108(29)/120 24 48

Virginia 80 80 20 20 34 34 Yes Yes 110/150 25 50

Washington 80 105.5 20 20 34 34 Yes Yes 103/156 22 43

West Virginia 80 80(30) 20 20 34 34 Yes Yes 104/110 20 45

Wisconsin 80 80 20 20 34 34 Yes-mod Yes-mod 100/191 20 60

Wyoming 117 117 20 20 36 36 Yes No 85/135 25 55

NS...Not specified WT...Weight table

Footnotes to table at end of Chapter.

Information sources: 

J. J. Keller & Associates, Vehicle Sizes and Weights Manual. July 1, 1994.

Specialized Carriers & Rigging Association (SC&RA), Permit Manual. July 19, 1994.

Western Association of State Highway and Transportation Officials (WASHTO), Guide for Uniform Laws and Regulations

Governing Truck Size and Weight. June 26, 1993.

11
12

National Network (NN) Other State Highways

State Length Kingpin Length Kingpin Overall

Alabama 57-0 41-0 KCRA(1) 53-0

Alaska 48-0 45-0 70-0

Arizona 57-6(7) 53-0 65-0

Arkansas 53-6 53-6

California 53-0 40-0 KCRTA(8)

38-0 KCSRA(9)

53-0 Same as NN

Colorado 57-4 57-4

Connecticut 53-0 48-0

Delaware 53-0 53-0 60-0

Dist. of Col. 48-0 48-0 55-0

Florida 53-0 41-0 KCRT(2) 53-0 41-0 KCRT

Georgia 53-0 41-0 KCRT 53-0 41-0 KCRT 67-6

Hawaii No Limit 45-0 60-0

Idaho 53-0 48-0 39-0 KCRA

Illinois 53-0 42-6 KCRA 53-0 42-0 KCRA

Indiana 53-0 40-6 KCRA 53-0 40-6 KCRA

Iowa 53-0 53-0 40-0 KCRA 60-0

Kansas 59-6 59-6

Kentucky 53-0 No Limit 57-9

Louisiana 59-6 No Limit 65-0

Maine 53-0(3) 43-0 53-0 65-0

Maryland 53-0(4) 41-0 KCRT 53-0 41-0 KCRT

Massachusetts 53-0(5) 53-0

Michigan 53-0 41-0 KCRT 50-0

Minnesota 53-0 41-0 KCRT 53-0 41-0 KCRT

Mississippi 53-0 53-0

Missouri 53-0(4) No Limit 60-0

Montana 53-0 53-0

Nebraska 53-0 53-0

Nevada 53-0 53-0 70-0

New Hampshire 53-0(6) 41-0 KCRT 53-0 41-0 KCRT

New Jersey 53-0 41-0 KCRT 53-0 41-0 KCRT

New Mexico 57-6 No Limit 65-0

New York 53-0(4) 41-0 KCRT 48-0 65-0

North Carolina 53-0 41-0 KCRT No Limit 60-0

North Dakota 53-0 53-0

Ohio 53-0 53-0

Oklahoma 59-6 59-6

Table 3. 1994 Maximum Semitrailer Lengths by State
13

Table 3. 1994 Maximum Semitrailer Lengths by State (cont.)

National Network (NN) Other State Highways

State Length Kingpin Length Kingpin Overall

Oregon 53-0 Varies

Pennsylvania 53-0 No Limit 60-0

Puerto Rico 48-0

Rhode Island 48-6 48-6

South Carolina 53-0 41-0 KCRT 48-0

South Dakota 53-0 53-0

Tennessee 53-0 41-0 KCRT 53-0 41-0 KCRT

Texas 59-0 59-0

Utah 53-0 40-6 KCRT 53-0 40-6 KCRT

Vermont 53-0(4) 41-0 KCRT 48-0 60-0

Virginia 53-0 37-0 Last

tractor axle to

first trailer axle.

No Limit 60-0

Washington 53-0 53-0

West Virginia 53-0 Same as VA No Limit 60-0

Wisconsin 53-0 41-0 KCRT No Limit 60-0

Wyoming 60-0 60-0

(1) KCRA = Kingpin to center of rear axle

(2) KCRT = Kingpin to center of rear tandem

(3) permit may be required

(4) Interstate and designated State routes

(5) Requires annual letter of authorization. Does not apply on the Massachusetts Turnpike

(6) Designated routes

(7) Only on Interstate System

(8) KCRTA = Kingpin to center of rearmost tandem axle

(9) KCSRA = Kingpin to center of single rear axle.

Figure 5 shows the States that presently allow various types of LCVs and Table 4 shows the

maximum allowable weights for two and three- trailer LCVs in the various States. It is clear from

this table that State LCV weight limits vary considerably. The ISTEA froze LCV weight limits at their

1991 levels; this freeze was extended in TEA-21.

Table 5 summarizes characteristics of various truck classes and how they currently are used. The table

shows that the 4 main types of LCVs -- eight-axle B-trains, Rocky Mountain doubles, turnpike doubles, and

triples -- are used in only a limited number of areas and currently have somewhat specialized uses.

An important assumption is that all States will adjust their TS&W limits to conform to the scenario limits

and that needed infrastructure improvements to accommodate all scenario vehicles will have been completed

including the construction of staging areas for certain LCVs. In practice, unless mandated to adopt Federal

TS&W limits, some States could be expected to retain their current limits or adopt less permissive limits

than the new Federal limits.
14

Pounds Truck Tractor and 2

Trailing Units

Truck Tractor and 3

Trailing Units

86.4 NM

90 OK OK

95 NE

105.5 ID, ND, OR, WA ID, ND,OR

110 CO CO

111 AZ

115 OH

117 WY

120 KS, MO 1 

123.5 AZ

127.4 IN, MA, OH IN

129 NV, SD, UT NV, SD, UT

131.06 MT

137.8 MT

143 NY

164 MI

 1 From Kansas, within 20 miles of border.

 Source: Final Rule on LCVs published in the Federal Register at 59 FR 30392 on June 13, 1994.

Table 4. Longer Combination Vehicle Weight Limits by State Highway

Networks

Developing the

networks upon

which certain LCVs

would be allowed to

operate was

difficult because

most States

currently do not

allow LCVs and

many States in the

Midwest and East

have indicated they

do not think LCVs

could operate safely

on their highways,

especially in and

around urban areas. 

Resource

constraints did not

permit analyzing

scenarios in which LCVs would be assumed to operate in certain regions of the country. For analytical

purposes it was assumed that LCVs would be allowed to operate on limited nationwide networks of

Interstate and other NHS routes. Figures 6 to 8 show the analytical networks assumed to be available for

different vehicles under the illustrative scenarios.

The NN shown in Figure 6 was designated by the Secretary in consultation with the States pursuant to the

STAA of 1982. A key factor in identifying routes to be included in this network was whether they could

accommodate the 48-foot semitrailer combinations and twin-trailer combinations that States are required to

allow on that network. 

Figures 7 and 8 show the illustrative networks assumed to be available for long double-trailer combinations

and triple-trailer combinations. The network for triples includes about 65,000 miles of highway including

some low volume two-lane highways in the West and some four-lane highways in the East that are not built

to Interstate standards. The longer doubles network contains about 42,500 miles of access-controlled,

interconnecting segments of the Interstate System and other highways of comparable design and traffic

capacity. 

Both networks are more extensive than some States would find acceptable, but both also exclude roads in

some States on which LCVs operate today, albeit with lower weights. In developing the analytical networks

the Department did not examine potential local constraints to LCV operations on those highways, but simply

selected highways that connect important markets. Resource constraints did not allow sensitivity analyses

of alternative networks to estimate how variations in network extent could
15

Configuration

Type

Number

of Axles

Common

Maximum

Weight 

(Pounds)

Current Use

Single-Unit

Truck

3 50,000

to

65,000

Single-unit trucks (SUT) are the most commonly

used trucks. They are used extensively in all

urban areas for short hauls. Three-axle SUTs are

used to carry heavy loads of materials and goods

in lieu of the far more common two-axle SUT.

4 or more 62,000

to

70,000

SUTs with four or more axles are used to carry

the heaviest of the construction and building

materials in urban areas. They are also used for

waste removal. 

Semitrailer 5 80,000

to

99,000

Most used combination vehicle. It is used

extensively for long and short hauls in all urban

and rural areas to carry and distribute all types of

materials, commodities, and goods.

6 or more 80,000

to

100,000

Used to haul heavier materials, commodities, and

goods for hauls longer than those of the four-axle

SUT. 

STAA

Double

5, 6 80,000 Most common multitrailer combination. Used

for less-than-truckload (LTL) freight mostly on

rural freeways between LTL freight terminals.

B-Train 

Double

8 105,500

to

137,800

Some use in the northern plains States and the

Northwest. Mostly used in flatbed trailer

operations and for liquid bulk hauls.

Rocky

Mountain

Double

7 105,500

to

129,000

Used on turnpikes in Florida, the Northeast, and

Midwest and in the Northern Plains and

Northwest in all types of motor carrier

operations, but most often it is used for bulk

hauls.

Turnpike

Double

9 105,500

to

147,000

Used on turnpikes in Florida, the Northeast, and

Midwest and on freeways in the Northern Plains

and Northwest for mostly truckload operations.

Triple 7 105,500

to

131,000

Used to haul LTL freight on the Indiana and Ohio

Turnpikes and in many of the most western States,

used on rural freeways between LTL freight

terminals.

Table 5. Characteristics of Typical Vehicles and How They Currently Are Used
16

States Allowing LCVs* States Allowing Triples

States Allowing Turnpike Doubles States Allowing Rocky Mountain Doubles

Figure 5. States Allowing Various Longer Combination Vehicles

Figure 6. National Network for STAA Vehicles
17

Figure 7. 1998 Comprehensive Truck Size and Weight Study Analytical Network for

Longer Combination Double-Trailer Vehicles

Figure 8. 1998 Comprehensive Truck Size and Weight Study

Analytical Network for Longer Combination Triple-Trailer Vehicles
18

affect potential LCV use. It is important to recognize that the networks developed for this study are purely

for analysis purposes and do not represent networks that the Department necessarily believes should or

could be used by LCVs, now or in the future.

Impact Assessment

Several factors must be considered when assessing potential TS&W options. Factors analyzed in previous

Departmental and TRB studies include infrastructure costs, safety, productivity, traffic operations, and 

intermodal competition. These same factors are evaluated in this study, although in more detail than has

been done in previous studies. 

Relationships between TS&W changes and these various impacts are complex and depend on specific

characteristics of vehicles anticipated to operate under various TS&W options. In general, changes in

allowable weights and dimensions intended to improve trucking productivity have adverse impacts on

infrastructure and most other impacts. While these impacts generally cannot be avoided, actions

can be taken to reduce the adverse impacts.

Infrastructure Costs

Pavement 

Potential pavement impacts associated with changes in TS&W regulations are of intense concern because of

the magnitude of Federal and State investments in pavement on our Nation’s highway systems. Many

factors contribute to pavement impacts that might occur following TS&W policy changes including

allowable axle load limits, changes in VMT by different vehicle classes, and changes in VMT and axle loads

on different highway classes. 

Table 6 shows the relative pavement damage caused by the different scenario vehicles analyzed in this

study. Pavement damage is expressed in terms of load equivalency factors per 100,000 pounds of cargo. 

This measure reflects both absolute pavement damage caused by each vehicle at the maximum weight at

which it can operate, as well as the benefits of moving the same volume of cargo in fewer trips. It also

shows that pavement impacts vary by type of pavement. Table 6 shows that pavement damage varies

depending on the specific vehicles and weights at which they are allowed to operate. Among the

combination vehicles, many can haul the same quantity of cargo as the five-axle semitrailer configuration

with less pavement damage, but relative damage depends on the types of axles on each vehicle (single,

tandem, or tridem) and the type of pavement upon which the vehicle is operating. Among the single unit

trucks, adding an axle can reduce pavement costs per unit of cargo carried for any of the configurations and

weights considered in this analysis. 

The analysis of impacts of each TS&W scenario on pavement costs uses the entire Highway Performance

Monitoring System (HPMS) sample in estimating system-wide pavement costs. This database of over

100,000 sample pavement sections is statistically representative of highways in each State, and thus captures

the effects of different pavement designs and types as well as changes in the volumes and weights of various

truck configurations under each TS&W scenario. In particular, the study evaluates the
19

contribution of 20 vehicle classes operating at a variety of different weights to 11 separate pavement

distresses based on pavement analysis methods developed for the 1997 Federal Highway Cost Allocation

Configuration

Gross

Vehicle

Weight 

(pounds)

Empty

Weight

(pounds)

Payload

Weight

(pounds)

No. Of

Vehicles 

per

100,000

pounds of

payload

Load Equivalency Factors***

Rigid

Pavement

Fatigue

(10-inch

thickness)

Flexible

Pavement (5-inch

wearing surface)

Fatigue Rutting

Three-Axle 

Single Unit Truck 54,000 22,600 31,400 3.18 13.4 17.8 13.0

Four-Axle

Single Unit Truck

64,000 26,400 37,600 2.66 9.6 14.4 12.2

71,000 26,400 44,600 2.24 9.2 14.6 11.2

Five-Axle

Semitrailer 80,000 30,500 49,500 2.02 5.7 9.3 10.3

Five-Axle

Semitrailer

(10-foot Spread)

80,000 30,500 49,500 2.02 6.3 12.2 10.9

Six-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

STAA Double

(five-axle) 80,000 29,300 50,700 1.97 8.3 9.9 9.7

B-Train Double

(eight-axle)

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

Rocky Mt.Double

(seven-axle) 120,000 43,000 77,000 1.30 7.8 9.9 9.5

Turnpike Double

(nine-axle) 148,000 46,700 101,300 0.99 5.0 7.7 7.2

Triple

(seven-axle)

114,000

(LTL )* 44,500 69,500 1.44 8.6 9.8 9.6

132,000

(TL )** 44,500 87,500 1.14 11.6 11.8 9.0

*LTL= Less-than-truckload 

**TL= Truckload 

***(based on 18,000-pound single axle with dual tires) 

Table 6. Theoretical Load Equivalency Factors per 100,000 pounds of Payload Carried

By Study Vehicle Configurations
20

Study. The distress models reflect the effects of single, tandem, and tridem axles at different weights on

different types of pavement. 

Bridges

Like pavement impacts, the impacts of TS&W policy changes on bridges depend on several factors

including the gross weight of the vehicle, the weight on various groups of axles, the distance between axle

groups, and the type and length of bridge. In previous TS&W studies by the Department and TRB, bridge

impacts have been among the most significant impacts of some TS&W policy changes. 

As noted above, the Federal Bridge Formula now controls vehicle weights to protect our Nation’s bridges. 

In particular it limits the weight on groups of axles depending on the distance between those axles. The two

most typical bridge designs are HS-20 which is common on higher class highways and H-15 which is typical

of bridges on lower class highways. The bridge formula is intended to assure that stresses placed on HS-20

bridges do not exceed the design stress by more than five percent and stresses on H-15 bridges are no more

than 30 percent greater than the design stress. Design stresses are well below stresses at which a bridge will

fail, but prolonged repetitions of high stresses can cause bridge deterioration to accelerate. 

The bridge formula is an approximation of the five percent and 30 percent overstress criteria discussed

above. The bridge analysis conducted for this study uses those criteria directly, estimating the stresses

imposed by different scenario vehicles on a sample of bridges from the National Bridge Inventory. If

stresses from scenario vehicles exceed the five percent or thirty percent criteria, those bridges are assumed

to require replacement. While previous studies by the Department and TRB have used slightly different

criteria to identify bridge deficiencies, they all have assumed that bridges found to be deficient would have

to be replaced. 

Comments to the docket for this study indicated that this assumption probably overestimates bridge costs. 

In practice some bridges could be strengthened and replacement of bridges on highways with low volumes

of the damaging vehicles perhaps would not have to be improved at all. Also, States might decide to “post”

bridges and not allow the heavier vehicles to use the bridges. 

In addition to estimating costs to replace all deficient bridges, the analysis also estimates additional user

delay and vehicle operating costs during the construction process. While highway agencies would not have

to pay these costs to allow scenario vehicles to operate, these user costs would represent significant costs

imposed on motorists. In urban areas the user costs may exceed bridge replacement costs.

Geometrics

The other major infrastructure impacts associated with TS&W changes are costs to upgrade geometric

deficiencies for different scenario vehicles. Geometric deficiencies are primarily interchanges and

intersections that cannot accommodate the turning radii of some scenario vehicles. The extent of geometric

deficiencies for different scenario vehicles was estimated based on a survey of interchange and intersection

design in nine States representing different regions of the country. For purposes of estimating improvement

needs it was assumed that no encroachment on shoulders or adjacent lanes would be allowed except for atgrade interchanges where vehicles would be allowed to encroach on one
21

lane in the same direction of travel. No costs are assumed for improvements needed to accommodate

existing vehicle configurations. 

Related to these geometric costs is the requirement that certain LCVs assemble and disassemble at staging

areas rather than being allowed to travel off the designated networks. Based on an analysis of existing

staging area operations on turnpikes and other factors, it was assumed that staging areas would be provided

every 15.6 miles in rural areas and at key points at the fringes of urban areas. 

In practice, staging areas could be provided in many ways with either public, private, or a combination of

public and private funding. Fewer staging areas might be required in some parts of the country but if fewer

staging areas were constructed, the ones that are constructed would likely have to be larger, and the average

distance to haul goods between origin, destination, and the staging areas would be greater. There is

considerable uncertainty concerning about exactly how staging areas would be implemented, and it is likely

that the provision of staging area services would evolve over time. Regardless of who pays for staging

areas, the cost of providing points where LCVs can assemble and disassemble is a cost that must be

considered in assessing the extent to which those vehicles would be used and the cost of operating those

vehicles. 

Safety Impacts

Crash rates

The safety analysis in this study includes an extensive review of past safety studies and a synthesis of results

that could be pulled from those studies. Extensive research into various aspects of truck safety has been

conducted over the years, but there still are many uncertainties about the safety of certain scenario vehicles. 

Reasons why it has been difficult to isolate effects of vehicle weights and dimensions on highway crash rates

include (1) weights and dimensions of vehicles involved in crashes often are not known or recorded on

accident reports; (2) even where data on the number of crashes for certain types of vehicles are known, the

VMT for those vehicles often is not known so it is difficult to develop crash rates for vehicles larger than the

typical vehicles in use; and (3) crash rates for larger vehicles in use in certain regions of the country or on

turnpike may not be transferrable to operations in other parts of the country where traffic volumes are

higher and the operating environment is less safe. Compounding difficulties in estimating crash rates for

certain scenario vehicles is the large switch to these vehicles from conventional truck configurations

estimated in the diversion analysis. The sheer volume of those larger and heavier vehicles would mean that

it would be much more difficult to regulate the use of those vehicles. All of these factors make predicting

crash rates for widespread use of certain scenario vehicles very problematic. 

Public Perception

Crash rates are perhaps the most important safety consideration, but other factors also must be factored into

assessments of the safety of certain scenario vehicles. One intangible factor is the public reaction to larger

and heavier trucks. While public perceptions may have little factual basis, they ultimately are important

factors affecting decisions concerning whether to allow such vehicles. As noted above, focus group

meetings were conducted to delve more deeply into driver perceptions of the safety of various vehicle

configurations in different operating environments. Truck drivers participating in the focus groups expressed

confidence that they could handle any larger trucks that might come along, but
22

questioned the need and desirability for larger trucks and said that maintaining safety would require changes

in highway conditions, training, equipment, and economic incentives. The vast majority of automobile

drivers participating in the focus groups indicated that they prefer the status quo and that if changes are

made they should be in the direction of greater restrictions on TS&W limits. Some indicated they could

accept a role for LCVs, but only under very strict limits and conditions. While opinions expressed in the

focus groups are not necessarily representative of all drivers, the focus groups do provide insights into

factors underlying opinions about the truck safety held by truck and automobile drivers. A working paper

prepared for this study summarizes focus groups findings in more detail. 

Vehicle Stability and Control

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 the relative safety of the vehicle compared to vehicles currently in widespread use. 

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. Perhaps the most

important vehicle stability property is susceptibility to rollover. Approximately 60 percent of crashes fatal to

heavy truck occupants involve rollovers. 

In general rollovers can result from one of two basic maneuvers – making a steady-state turn at too high a

speed or high speed evasive maneuvers. Virtually all vehicles are susceptible to rolling over, but heavy

trucks are especially susceptible. The principal attributes that affect a vehicle’s rollover tendencies are the

height of the center of gravity (cg) of the cargo, and the vehicle’s track width, suspension, and tire

properties. 

A measure of a vehicle’s propensity to rollover during a steady-state turn is its static roll stability (SRS). 

The SRS is measured in terms of the lateral acceleration (g forces) required to lift a wheel off the ground. 

The higher the SRS, the less susceptible the vehicle is to rollover. The typical 80,000-pound tractorsemitrailer has a SRS of about 0.3 gs compared to 0.8 gs or higher for automobiles.

Rollovers that occur as the result of evasive maneuvers are associated primarily with multitrailer

combinations, but other trucks with high centers of gravity can also roll over when making quick evasive

maneuvers. The number of articulation points on multitrailer combinations significantly affect this kind of

rollover because they accentuate the “crack-the-whip” phenomenon where rapid steering maneuvers made

in the tractor can be amplified by factors of two or three as they get to the rear trailer. Seemingly benign

maneuvers by the tractor can result in the rearmost trailer skidding sideways into adjacent lanes, or worse,

rolling over.

Several vehicle attributes can contribute to rollover during evasive maneuvers. As noted above, the more

articulation points in the combination, the greater the susceptibility to rollover. Tractor-semitrailers have a

single articulation point, doubles typically have three articulation points, and triples usually have five. 

Second, the shorter the wheelbase lengths of the trailers in the combination the more susceptible the vehicle

is to rollover. Finally, the lower the SRS of each trailer in the combination, the more susceptible the vehicle

is to rollover during evasive maneuvers.
23

Two measures characterize a vehicle’s susceptibility to rollover during evasive maneuvers, the rearward

amplification factor and the load transfer ratio. The rearward amplification factor is the ratio of the lateral

acceleration of the rearmost trailer to the lateral acceleration of the tractor when making rapid steering

movements. Tractor-semitrailer combinations have a factor of 1 and STAA doubles a factor of 1.7. In

general a rearward amplification factor of 2 or less is considered acceptable.

The load transfer ratio is a measure of the dynamic roll stability of a truck. It measures the proportion of a

vehicle’s total axle load that is carried on one side of the truck relative to the other. A perfectly balanced

vehicle would have a load transfer ratio of 0.5, while a vehicle with all its weight on one side of the vehicle

(and the other side in the air) would have a load transfer ratio of 1.0. The Society of Automotive Engineers

has developed a standard evasive maneuver for evaluating vehicle dynamic stability. Load transfer ratios

for each scenario vehicle can be calculated based on this standard evasive maneuver to determine which

vehicles are more likely to roll over under that maneuver.

Vehicle Comparisons

Figure 9 compares the stability and control of scenario vehicles with a reference vehicle, the conventional

five-axle tractor-semitrailer combination. For each of the three measures discussed above, the percentage

difference between the scenario vehicle and the reference vehicle is shown. 

Only two vehicles in Figure 9 have better static roll stability than the reference vehicle, the two STAA

doubles. The worst vehicles are the three single unit trucks because of their high centers of gravity. Each

of the other vehicle classes is within 10 percent of the five-axle tractor-semitrailer. 

Rearward amplification shows quite different relationships among the scenario vehicles. The three single

unit trucks all have less rearward amplification than the reference vehicle as do the two six-axle tractorsemitrailers. The multitrailer combinations with short trailers all have considerably worse rearward

amplification than the reference vehicle with triples being the worst vehicle class. The benefits of reducing

the number of articulation points can be seen by comparing the “C-train” triples and “C-train” doubles with

“A-train” triples and doubles respectively. The A-train configurations use a single drawbar to connect the

sets of trailers while the C-trains have double drawbar connections that reduce the number of articulation

points. The “B-train” is yet another type of connection between two trailers where the front trailer has a

permanently attached “fifth-wheel” connected to its frame. The eight and nine-axle doubles have more

rearward amplification than the reference tractor-semitrailer, but considerably less than the other multitrailer

combinations. Their longer trailer lengths and the B-train connection for the eight-axle double contribute to

their better performance.

Differences in load transfer ratios between the reference tractor-semitrailer and the scenario vehicles show

that many of the scenario vehicles would likely rollover under SAE’s standard evasive maneuver, including

the conventional STAA double and the 3-axle single unit truck. Multitrailer combinations with B and C-train

connections and the six-axle tractor-semitrailer were the most stable of the scenario vehicles. 

Traffic Operations

Changes in the weights, dimensions, and volumes of trucks in the traffic stream resulting from TS&W

changes could affect other motorists using the highway. Among the effects that could be anticipated are
24

Rearward Amplification

16.74

16.74

16.74

-79.93

-46.21

1.58

0.67

-71.52

-22.4

-23.98

-212.07

-99.67

-35.89

-250 -200 -150 -100 -50 0 50

3-Axle Truck at 54,000 pounds

4-Axle Truck at 64,000 pounds

4-Axle Truck at 71,000 pounds

5-Axle STAA A-Train Double at 80,000 pounds

5-Axle STAA C-Train Double at 80,000 pounds

6-Axle Semitrailer at 90,000 pounds

6-Axle Semitrailer at 97,000 pounds

7-Axle Rocky Mountain Double at 120,000 pounds

8-Axle B-Train Double at 124,000 pounds

8-Axle B-Train Double at 131,000 pounds

7-Axle A-Train Triple at 132,000 pounds

7-Axle C-Train Triple at 132,000 pounds

9-Axle Turnpike Double at 148,000 pounds

Load Transfer Ratio

-87.27

-62.92

-80.71

-87.27

-1.87

3.37

-1.5

-87.27

6.37

1.87

-87.27

27.34

-49.06

-100 -75 -50 -25 0 25 50

3-Axle Truck at 54,000 pounds

4-Axle Truck at 64,000 pounds

4-Axle Truck at 71,000 pounds

5-Axle STAA A-Train Double at 80,000 pounds

5-Axle STAA C-Train Double at 80,000 pounds

6-Axle Semitrailer at 90,000 pounds

6-Axle Semitrailer at 97,000 pounds

7-Axle Rocky Mountain Double at 120,000 pounds

8-Axle B-Train Double at 124,000 pounds

8-Axle B-Train Double at 131,000 pounds

7-Axle A-Train Triple at 132,000 pounds

7-Axle C-Train Triple at 132,000 pounds

9-Axle Turnpike Double at 148,000 pounds

Static Roll Stability

-23.05

-11.21

-16.2

2.18

2.8

-0.31

-4.05

-5.92

-0.31

-3.12

-7.17

-7.79

0

-30 -20 -10 0 10

3-Axle Truck at 54,000 pounds

4-Axle Truck at 64,000 pounds

4-Axle Truck at 71,000 pounds

5-Axle STAA A-Train Double at 80,000 pounds

5-Axle STAA C-Train Double at 80,000 pounds

6-Axle Semitrailer at 90,000 pounds

6-Axle Semitrailer at 97,000 pounds

7-Axle Rocky Mountain Double at 120,000 pounds

8-Axle B-Train Double at 124,000 pounds

8-Axle B-Train Double at 131,000 pounds

7-Axle A-Train Triple at 132,000 pounds

7-Axle C-Train Triple at 132,000 pounds

9-Axle Turnpike Double at 148,000 pounds

Percent Change

Figure 9. Comparison of Stability and Control Measures for Scenario 

Vehicles Relative to Five-Axle Tractor Semitrailer
25

changes in congestion levels, traffic interference associated with the offtracking of longer vehicles, and

additional distance required to pass longer vehicles. Other impacts include potentially worse acceleration of

heavier trucks unless they had more powerful engines, interference at intersections if trucks cannot stay

within their lanes, and decreased visibility of traffic signs because of longer trucks. Impacts of most of these

potential factors are very difficult to quantify because they are site specific.

The one operational impact that was quantified in this study is potential effects on congestion. Traffic

congestion occurs when the volume of traffic using a highway at a particular time approaches or exceeds the

capacity of the highway. Capacity is usually measured in terms of the maximum number of automobiles

that can pass a given point on a roadway during a given period of time under existing conditions. Many

factors affect highway capacity including highway design factors (access control, number of lanes, lane

width, curves, grades and other factors), lateral clearance from objects at the side of the road, weather, and

the presence of trucks in the traffic stream. 

Impacts on Capacity

The effects of trucks on capacity depends on roadway and traffic conditions. For instance, the effect of

trucks is greater in mountainous terrain with long, steep grades than it is on flat terrain because trucks

typically cannot maintain their speed on such grades as well as passenger vehicles. Likewise the effect of

trucks on capacity is greater on two-lane roads than freeways, especially when there is limited sight distance

for passing. 

The effect of trucks on capacity is measured in terms of their “passenger-car equivalents (PCEs).” On level

terrain a truck may have the effect of two passenger vehicles on capacity, but in mountainous terrain the

PCE may be eight. Most previous studies have used a single typical truck to evaluate highway capacity. 

This study and the companion 1997 Federal Highway Cost Allocation Study extended the state-of-the-art in

estimating PCEs by evaluating relative effects of many different types of trucks. The primary variables

affecting the relative PCEs of different trucks are their length and their weight-to-horsepower ratio, which is

a measure of their ability to accelerate.

Tables 7 and 8 show PCEs for trucks with different lengths and weight-horsepower ratios in rural and urban

areas. In both rural and urban areas the length of the vehicle has only a minor effect on PCEs, and that

effect is little more than the fact that the longer vehicle occupies more space. On very congested roads with

many closely spaced interchanges and high volumes of very long trucks it is likely that the effect of those

trucks on traffic flow would be greater than shown in these tables because of they would interfere with

merging movements at the on- and off-ramps. The weight-to-horsepower ratio has a greater effect on

traffic flow, especially in rural areas. 

Table 7 also dramatically shows the effect of grade on PCEs. On a four-lane rural Interstate PCEs for 80-

foot long trucks can range from 2.6 to over 14 depending on the grade and weight-to-horsepower ratio. On

two-lane highways PCEs can be even higher.

Table 9 summarizes effects of TS&W characteristics on various elements of highway and traffic operations.
26

Energy and Environment

Environmental impacts of highway travel, especially impacts on air quality and health, have been

increasingly important considerations for decision makers at all levels of government. Research into detailed

factors that affect emissions from different truck classes has lagged research on factors affecting automobile

emissions. There are several types of emissions from trucks that contribute to health problems and other

impacts on persons and property. They include nitrogen oxides, particulate matter, volatile organic

compounds, and sulfur oxides. Mechanisms by which these emissions ultimately affect health are complex

and in some cases poorly understood at present. 

Analytical models used by the Environmental Protection Agency (EPA) and many other environmental

researchers do not differentiate among the truck classes of interest in this study. Emissions from heavy

trucks vary directly with VMT, but other factors such as relative fuel economy may also affect emission

rates. Further research will be required to develop factors that relate emissions to vehicle weights and

dimensions. 

While a primary concern about the relative energy consumption under alternative TS&W policies is the

connection between energy consumption and emissions, the relative energy consumption of different

Roadway

 Type

Grade

Vehicle Weight to 

Horsepower Ratio

(pounds/horsepower

)

Truck Length

(feet)

Percen

t

Lengt

h

(miles

)

40 80 120

FourLane

Interstate

0 0.50

150 2.2 2.6 3.0

200 2.5 3.3 3.6

250 3.1 3.4 4.0

3 0.75

150 9.0 9.6 10.5

200 11.3 11.8 12.4

250 13.2 14.1 14.7

TwoLane

Highway

0 0.50

150 1.5 1.7 Not Simulated

200 1.7 1.8 Not Simulated

250 2.4 2.7 Not Simulated

4 0.75

150 5.0 5.4 Not Simulated

200 8.2 8.9 Not Simulated

Table 7. Vehicle Passenger Car Equivalents -- Rural Highways
27

types of trucks and alternative modes of freight transportation remains a policy issue. Table 10 shows the

relative miles per gallon of fuel consumed for different truck configurations at different weights. 

Truck noise comes from three sources, the engine, the exhaust, and the tires. Truck noise begins to

dominate noise from other vehicles in the traffic stream once trucks account for more than three percent of

the traffic stream.

Roadway 

Type

Traffic Flow 

Condition Grade

Vehicle to

 Horsepower Ratio

(pounds/horsepower)

Truck Length

40 80 120

Interstate

Congested 0

150 2.0 2.5 2.5

200 2.5 3.0 3.0

250 3.0 3.0 3.0

Uncongested 0

150 2.5 2.5 3.0

200 3.0 3.5 3.5

250 3.0 3.5 4.0

Freeway and 

Expressway

Congested 0

150 1.5 2.5 2.5

200 2.0 2.5 2.5

250 2.0 3.0 3.0

Uncongested 0

150 2.0 2.0 2.0

200 2.5 2.5 2.5

250 3.0 3.0 3.0

Other 

Principal 

Arterial

Congested 0

150 2.0 2.0 2.5

200 2.0 2.0 3.0

250 3.0 3.0 4.0

Uncongested 0

150 3.0 3.0 3.5

200 3.5 3.5 3.5

250 3.5 4.0 4.0

Table 8. Vehicle Passenger Car Equivalents -- Urban Highways
28

Noise passenger car equivalency factors were developed for this study based on FHWA noise prediction

models. Those models do not differentiate among truck types so a generalized factor for all heavy trucks

was developed.

The measure that traditionally has been used to estimate the economic costs associated with transportationrelated noise is the effect of that noise on residential property values. A synthesis of property value studies 

was conducted for the 1997 Federal Highway Cost Allocation Study and results are used to estimate noise

related costs associated with TS&W scenarios. Because there currently are no

Vehicle Features Traffic 

Congestion

Vehicle

Offtracking Traffic Operations

Low

Speed

High

Speed Passing

Acceleration

(merging and

hill climbing)

Lane

Changing

Intersection

Requirements

Size

Length - e - E + e - E — - E - E

Width — - e + e - e — - e —

Height — — - e — — — —

Design

Number of 

units — + E - E — — - e —

Type of 

hitching — + e + E — — + E —

Number of 

Axles — + e + e — — + e —

Loading

Gross vehicle

weight - e — - E - E - E - e - E

Center of 

gravity height — — - e — — - e —

Operation

Speed + E + E - E - E — + e + E

Steering 

input — - E - E — — - E —

+/- As parameter increases, the effect is positive or negative.

E = Relatively large effect. e = relatively small effect. -- = no effect.

Table 9. Summary of Effects of Truck Size and Weight Characteristics

on Highway and Traffic Operations
29

data on the relative differences in

noise for different types of trucks,

changes in truck volumes and

speeds are the most significant

factors affecting noise impact

estimates.

Table 11 shows that equivalency

factors are much greater at low

speeds than at high speeds, varying

from 85 at 20 miles per hour to 14

at 60 miles per hour.

Rail Impacts

Motor carriers, railroads, barges, and pipelines are the principal transportation modes for moving intercity

freight, with motor carriers and railroads having the largest market shares in both revenues and tonnage. 

Railroads have a competitive advantage over motor carriers in hauling bulk commodities, and trucks have an

advantage in hauling low density, high-value commodities. But railroads and trucks compete for many

movements, especially the increasing volume of intermodal traffic.

Since the Staggers Rail Act was passed in 1980 the railroads have been reorganizing to make their

operations more efficient and profitable. There is concern that, if changes in TS&W regulations allowed

larger trucks, those trucks would draw freight from the railroads and adversely affect their profitability. 

Configurations

Gross Vehicle Weight (pounds)

40,000 60,000 80,000 100,000 120,000 140,000

Three-axle Single-Unit Truck 5.11 4.42

Four-axle Single-Unit Truck 4.80 4.15

Five-axle Tractor-Semitrailer 5.44 4.81 4.31

Six-axle Tractor-Semitrailer 5.39 4.76 4.27

Five-axle STAA Double 5.95 5.29 4.76

Seven-axle Rocky Mountain

Double 5.08 4.58 4.36 4.16

Eight-axle (or more) Double 5.08 4.82 4.58 4.36

Triple-Trailer Combination 5.29 5.01 4.76 4.54

Source: FHWA

Table 11. Diesel Fuel Use Rates for Illustrative Vehicle at Different Weights

(miles per gallon)

Vehicle

type

Speed (miles per hour)

20 30 40 50 60

Auto 1.0 1.0 1.0 1.0 1.0

Truck 85 44 27 19 14

Source: FHWA

Table 10. Noise Passenger Car Equivalents for Trucks
30

A shrinking volume of traffic is of particular concern to railroads because railroads are a decreasing cost

industry. They have high fixed and common costs, and per-unit costs decline as production increases to

capacity. If production (traffic volume) decreases, however, the high fixed costs must be spread over the

lower volume, resulting in higher per-unit costs. The railroads could price some shipments below average

total cost, but above variable cost, to retain traffic, but any such discounts would have to be made up

through higher costs to captive shippers, increasing total freight costs for those shippers.

Impacts of the various illustrative TS&W scenarios are estimated using the Department’s Intermodal

Transportation and Inventory Cost (ITIC) model and an Integrated Financial Model. Data limitations

prevented analyzing potential truck-to-rail diversion under the two scenarios that would have rolled back

Federal TS&W limits, but potential rail-to-truck diversion resulting from scenarios that assumed increases in

allowable vehicle weights and dimensions are estimated. 

The ITIC model uses the Surface Transportation Board Carload Waybill Sample of rail freight shipments

and estimates total shipper transportation and logistics costs for each record in that database. Comparable

transportation and logistics costs to move the same shipments by truck operating under the TS&W limits

assumed in each illustrative scenario are estimated and compared with the rail costs. If trucks costs are

lower, the shipment is assumed to shift from rail to truck. A more detailed description of the diversion

model is contained in Volume III of this report.

In addition to estimating the total diversion of rail traffic to trucks under the illustrative TS&W scenarios,

the ITIC model can estimate (1) remaining rail revenues after accounting for losses due to diversion and

discounting to hold traffic and (2) the car miles remaining on the railroads. 

Using these ITIC outputs as inputs, the Integrated Financial Model uses the change in revenues and the

estimated change in railroad freight service expense for remaining car miles to measure the impact on the rail

industry’s financial condition following changes in TS&W regulation. Post-diversion return on investment is

calculated along with the increase in rail rates that would be required to return the rail industry to its prediversion financial condition. These estimates are made for the rail industry as a whole and for four

individual railroads

Shipper Costs

Previous TS&W studies by the Department and TRB have concluded that certain increases in allowable

vehicle weights and dimensions could reduce the costs of shipping freight, thereby increasing productivity. 

Not every shipment would benefit from increases in either the allowable weight or cubic capacity of a truck. 

Time sensitive shipments that are becoming increasingly important with just-in-time and other advanced

logistics systems might not benefit from larger trucks. Likewise, short-distance moves that have either an

origin or destination away from roads on which larger vehicles are allowed to operate might not benefit. But

studies have shown that many freight transportation markets potentially could benefit from increases in

allowable truck weights and dimensions. The extent of the benefit would depend on details of changes in

TS&W regulations and responses by States, different segments of the trucking industry, and shippers to

such changes. 

Shippers are concerned about more than just the cost of moving goods between origin and destination when

they make transportation-related decisions. They and their customers also consider other logistics costs

including inventory, product packaging, plant location, production processing requirements. For
31

instance if a shipper wants to reduce transportation costs by using larger trucks, it might incur higher

inventory costs to store the larger quantities of goods that can be shipped in the larger trucks, and the

shipper’s customers might also incur higher inventory costs to store goods at the destination. Just-in-time

inventory systems work with little or no inventories so larger shipments in larger trucks might not suit firms

that use such inventory systems, depending on the nature of the product.

The ITIC model estimates both transportation and key inventory costs associated with moving goods by rail

or by various truck configurations. For a given change in TS&W limits, the model predicts whether

changes in transportation and inventory costs would cause a given shipment to be transported by an

alternative mode or truck configuration. As noted above the Carload Waybill Sample is used for rail

shipments. The database of truck shipments comes from surveys of shipments at truck stops. If total

transportation and logistics costs are estimated to be lower for an alternative mode or truck configuration,

the shipment is assumed to divert to that alternative. 

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 could operate, and

the networks of highways

upon which scenario

vehicles could operate. 

Those illustrative scenarios

are briefly described below.

Uniformity Scenario

Figure 10 shows the major

scenario vehicles for the

Uniformity Scenario and

key analytical assumptions

underlying the scenario. 

Tables 2, 3,and 4 showed

Three-axle single unit truck

Maximum weight – 51,000 pounds

Five-axle tractor-semitrailer

Maximum weight – 80,000 pounds

Five-axle STAA double

Maximum weight – 80,000 pounds

Main Features

• Extend Federal gross

vehicle weight and axle

weight limits to the

National Network for

STAA vehicles.

• Grandfather provisions

removed.

Available Highways

• National Network for

STAA vehicles

Access Provisions

• Current Federal and State

provisions

Figure 10. Uniformity Scenario
32

the large variation in State TS&W

limits off the Interstate System

and the many States that have

grandfathered weight limits on the

Interstate System. This scenario

assumes that grandfather

provisions in current Federal law

would be removed and that States

would be required to adopt

Federal weight limits on all NN

highways. States now exercising

grandfather rights to allow heavier

vehicles on the Interstate System

would have to roll those weights

back to the current Federal limits. 

They also would have to roll back

higher limits they may have on

other NN highways. With an

80,000- pound weight limit, LCVs

would be impractical for all but

the lightest loads. 

The STAA of 1982 was intended

to improve the uniformity of

TS&W regulations and to extend

regulations beyond the Interstate

System. However, it did not

require any rollback of State

TS&W limits. Rather it

established minimum weights and

dimensions that would have to be allowed on certain highways.

Several States currently 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. Nondivisible load permits would continue. Off the NN, vehicles would continue to operate at current Stateregulated weights. 

North American Trade Scenarios

The North American Trade Scenarios assume 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 and the second at 51,000 pounds. 

Figure 11 shows key assumptions for these scenarios. The 51,000-pound limit would allow six-axle tractorsemitrailers to operate at 97,000 pounds which would permit transportation of international containers

loaded to the International Standards Organization (ISO) limit. Other vehicles considered in this scenario

are a four-axle single unit truck weighing up to 71,000 pounds and an eight-axle B-train double weighing up

to 131,000 pounds with trailer lengths of 33 feet. Because they corner as well as current tractor-semitrailers

and are relatively stable vehicles, the eight-axle B-train double is

Four-axle single unit truck

64,000 pounds or 71,000

pounds maximum weight

Six-axle tractor-semitrailer

90,000 pounds or 97,000

pounds maximum weight

Eight-axle B-train double

124,000 pounds or 131,000

pounds maximum weight

Main Features

• Combination vehicles

widely used in Canada

and Mexico

• Introduces tridem-axle

weight limits

Available Highways

• Current National

Network for STAA

vehicles

Access Provisions

• Current Federal and

State provisions

Figure 11. North American Trade Scenarios
33

7-axle Rocky Mountain Double

Maximum weight – 120,000

pounds

9-axle Turnpike Double

Maximum Weight – 148,000 pounds

8-axle B-train double

Maximum weight – 124,000 pounds

(33-foot trailers)

Triple-trailer combination

Maximum weight – 132,000 pounds

Main Feature

• Broad national LCV

operations

Available Highways

• RMDs and TPDs – 42,000

mile analysis network

• Triples – 60,000 mile

analysis network

• 8-axle B-train double –

National Network for STAA

vehicles 

Access Provisions

• RMDS and TPDs – none off

the analysis network

• Triples – State issued

permits

• 8-axle B-train doubles –

current Federal and State

provisions

Figure 12. Longer Combination Vehicles Nationwide Scenario

assumed to 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

5 States east of the Mississippi River. The ISTEA froze LCV operations, preventing their use in States

where they were not permitted on June 1, 1991. This freeze was extended in TEA-21. As indicated in

Figure 12, the LCVs Nationwide Scenario assumes LCV operations on a nationwide network. Limited

networks would be designated upon which LCVs could operate. Turnpike doubles (twin 53-foot trailer

combinations weighing up to 148,000 pounds) and Rocky Mountain Doubles (combinations with one 53-

foot trailer and one 28.5-foot trailer weighing up to 120,000 pounds) 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; travel off the network would be in single trailer combinations. Triple-trailer

combinations (combinations with three 28.5-foot trailers weighing up to 132,000 pounds) and eight-axle

twin-trailer

combinations with

two 33-foot trailers

weighing up to 

124,000 pounds

would be allowed to

travel off their

networks to get to

origins and

destinations under

current access rules

because they can

negotiate curves as

well as current

tractor-semitrailer

combinations. In

practice triple trailers

and the eight-axle

twin trailers might

not be allowed this

degree of access off

their designated

networks, but there

was no way to

estimate the extent to

which access might

be granted. To the

extent that diversion

to those two vehicles
34

may be overestimated, all of the

impact measures, both positive

and negative, are also

overestimated. The scenario

assumes that all States would

uniformly adopt the new limits,

and therefore captures the

maximum impact. All other

Federal size and weight controls

would remain. 

H.R. 551 Scenario

H.R. 551, “The Safe Highways

and Infrastructure Preservation

Act,” was introduced in 1994

during the 103rd Session of

Congress, and again in 1997 during the 105th Session. The bill would federalize certain areas of truck

regulation 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, and (3) it would freeze weight limits (including divisible load permits) on non-Interstate 

portions of the NHS. 

Triples Nationwide Scenario

This scenario assumes the

operation of triple-trailer

combinations across the

country at the same weights

and dimensions as are

assumed under the Scenario. 

Figure 14 gives key

assumptions of this Scenario. 

Illustrative Scenario

Impacts

Diversion

Table 12 shows estimates of

traffic diversion from existing

trucks and from rail to

selected vehicles for each of

the scenarios. Total vehicle

miles of travel

2-S1-2-2

Seven-axle triple-trailer combination

132,000 pounds (maximum)

Main Feature

ü Broad national operation

of triple-trailer

combinations and new

weight limits for

triple-trailer combinations

Available Highways

ü 65,000-mile system

Access Provisions

ü State issued permits

Figure 13. Triples Nationwide Scenario

SU3

3-S3

“STAA” 2-S1-2

SU4

Two- to four-axle single unit truck

Current law at 54,000 pounds to 70,000 pounds

Five- to six-axle semitrailer

Current law at 80,000 pounds to 100,000 pounds

Five to six-axle STAA double trailer combination

Current law at 80,000 pounds

Main Features

ü Phases in elimination of

semitrailers over 53 feet

long

ü Assumes status quo

weights

Available Highways

ü National Highway System

Access Provisions

ü Current Federal and State

provisions

Figure 14. H.R. 551 Scenario
35

(VMT) do not equal the sum of VMTs for individual vehicle classes because not all vehicle classes are

shown. It should be noted that base case VMT exceeds current VMT because the analysis year is 2000;

growth in the economy through 2000 will increase truck VMT. 

The two illustrative scenarios involving some rollback of State TS&W limits show small increases in 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 those two vehicle classes. 

The four scenarios that assume heavier vehicle weights all show large (greater than 70%) 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 ten 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 since the twins are assumed to have virtually

unlimited access off the NN and have significantly greater cubic capacity and payload. 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. 

The relative diversion to eight-axle twins compared to six-axle tractor-semitrailers is larger than has been

experienced in Canadian Provinces where similar configurations are allowed. Several comments to the

docket expressed concern that estimated diversion to the eight-axle twins is too high in these scenarios. 

Certainly it would take time for shippers and carriers to learn how to efficiently use such a vehicle and to

manage fleets with multiple trailer types, but with the widespread access assumed for eight-axle twins in this

scenario and the large cubic capacity, substantial transportation cost savings could be possible. If States did

not provide the liberal access assumed in this study, or if cargo handling and other logistics costs associated

with using the eight-axle twins were larger than assumed, diversion would be lower. The various

assumptions in these scenarios are discussed in detail in Volume III of the report. 

Estimated reductions in total VMT under the two LCV scenarios are about twice as great as under the North

American Trade Scenarios. In addition to diverting large volumes of traffic currently shipped in five-axle

tractor-semitrailers, LCVs could also divert less-than-truckload traffic currently being shipped in STAA

doubles. Even in the Triples Nationwide Scenario, considerable truckload traffic is diverted from five-axle

tractor-semitrailers because of the greater cubic capacity and gross weight of the triple. While little

truckload traffic currently moves in triples, the liberal access and high gross weight limit assumptions in the

scenario result in a vehicle that has relatively low costs per payload ton-mile. If access were more

restricted, as would be likely in many States, the allowable gross weight lower, and the handling and other

logistics costs associated with using triples higher than are assumed in this scenario, the diversion to triples

would be lower than shown in Table 12. Assumptions in these scenarios are discussed in detail in Volume

III.

Costs

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 13 shows estimated changes from

base case levels for key impact areas. Changes are expressed in terms of cost changes for each of the

impact areas except rail contribution. The change in rail contribution is a measure of the amount of revenue

available to cover rail fixed costs after variable costs have been covered.
36

Table 13 indicates that 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 the Department and TRB, but it may overestimate bridge-related costs based on

comments by several States. In practice, depending on the degree of overstress, the volume of vehicles

expected to utilize the bridge, and 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. Lightly traveled bridges that were

significantly overstressed might simply be posted to prevent the most damaging vehicles from using the

bridges. Posting significant numbers of bridges, however, would affect the level of utilization of prohibited

vehicle classes. Impacts of heavy trucks on fatigue and bridge deck deterioration are not estimated. An ongoing 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. Freeway interchanges and at-grade intersections would have to be modified to accommodate

some longer vehicles. In scenarios analyzed for this study, turnpike doubles and Rocky Mountain doubles

are assumed to be restricted to the limited networks upon which they can operate because of their long

turning radii. Staging areas are assumed to be required to allow those vehicles to assemble and disassemble. 

In some Western States those vehicles can travel more widely than is assumed in the illustrative scenarios,

but the vehicles operating in those States are shorter and have lower weight limits 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 the Department and TRB, this study estimates that certain scenarios could

produce significant reductions in shipping costs. Changes in shipping costs shown in Table ES-2 are all

smaller in percentage terms than changes in some other impacts, but 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 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 over 50 percent under the LCVs Nationwide Scenario and by somewhat lesser amounts under

the North American Trade and Triples Nationwide Scenarios, which also allow nationwide LCV operation. 

Volume III contains estimates of changes in rail contribution for several individual railroads for each

scenario. If allowable vehicle weights and dimensions were reduced, as assumed in the Uniformity Scenario

impacts on rail contribution would be smaller.

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. Previous TS&W

studies have estimated changes in crashes and crash costs that might result from TS&W changes, but in this

study the Department determined that changes in crash rates could not reliably be estimated for the
37

Vehicle Class 5-axle tractorsemitrailer6-axle tractorsemitrailerLCVs Total Truck 2 Rail

Illustrative Scenario VMT %

change

VMT %

change

VMT %

change

VMT %

change

Carmiles% change

Base Case 83,895 na 6,059 na 1,517 na 128,288 na 25,555 na

Uniformity 91,205 8.7 3,519 -41.9 542 -64.3 132,351 3.2 na na3

N.A. Trade (1) 22,274 -73.5 6,209 2.5 49,837 3185.2 114,671 -10.6 24,354 -4.7

N.A. Trade (2) 24,997 -70.2 6,246 3.1 47,453 3028.1 114,632 -10.6 24,073 -5.8

LCV nationwide 19,611 -76.6 na1 na1 40,980 2601.4 98,562 -23.2 20,546 -19.6

H.R. 551 83,915 0.0 6,051 -0.1 1,517 0.0 128,311 0.0 na na

Triples 23,405 -72.1 na1 na1 39,647 2513.5 102,400 -20.2 24,533 -4.0

N.A. Trade (1) – 44,000 pound tridem axles; N.A. Trade (2) – 51,000 pound tridem axles.

1

 To facilitate the diversion analysis, six-axle tractor-semitrailers were not included in the analysis for the two scenarios involving LCVs.

2

 The Total does not equal the sum of the three vehicle classes shown in the table because other vehicle classes included in the Total are not

shown in the table.

3

 Potential diversion from truck to rail under the Uniformity and H.R. 551 Scenarios could not be estimated because of lack of data on rail

pricing.

Table 12. Estimated Diversion for Selected Vehicle Configurations 

for Illustrative Truck Size and Weight Scenarios
38

Uniformity N.A. Trade (1) N.A. Trade (2) LCV Nationwide H.R. 551 Triples

Pavement Costs -0.3 -1.6 -1.2 -0.2 0 0

Bridge Costs -13.0 +33.1 +42.2 +34.4 0 +10.4

Geometric Costs 0 +13.3 +13.3 +965.0 0 0

Congestion Costs +0.6 -1.2 -1.2 -2.9 0 -7.6

Energy Costs +2.1 -6.2 -6.3 -13.8 0 -12.8

Shipper Costs +3.0 -5.1 -7.0 -11.4 0 -8.65

Rail Contribution

1 na -42.8 -49.7 -55.8 na -38.2

N.A. Trade (1) – 44,000 pound tridem axles; N.A. Trade (2) – 51,000 pound tridem axles.

1

 The amount of rail revenue available to pay fixed costs after freight service (variable) costs have been covered.

Table 13. Estimated Impacts of Illustrative Truck Size and Weight Scenarios

(Percent Change from Base Case)
39

LCV scenarios. The small body of evidence on LCV crash rates in Western States are based on such

different operating conditions and vehicles than those evaluated in this study that they do not provide a

credible basis for estimating crash rates for vehicles with the dimensions and weights analyzed in this study,

especially on congested highways on eastern portions of the illustrative LCV networks. Other factors

considered in assessing safety impacts of possible TS&W changes are stability and control properties of

different configurations, and perceptions of drivers concerning the safety of longer and heavier vehicles. 

The LCV configurations generally show poorer stability or control properties than the base tractorsemitrailer configuration. Short multitrailer combinations have poor lateral stability that can result in the

rearmost trailers traveling outside their lane or, at the extreme, rolling over if rapid steering maneuvers are

required. In general the shorter the trailers, the worse the lateral instability, although certain types of trailer

connections can reduce this instability. Thus while shorter trailers on triple trailer combinations reduce

offtracking, they also reduce lateral stability. Reducing allowable weights and dimensions of scenario

vehicles would improve stability and control, but would also reduce productivity benefits. Volume III

presents detailed results of safety-related analyses conducted for this study.

Future Research

A review by TRB of data and methods used in this study was initiated but put on hold while TRB conducts

the TS&W study called for in TEA-21. The intent of this review is to help develop a long-term research

agenda to continuously improve the Department’s ability to estimate impacts of alternative TS&W policy

options. The TRB review is expected to get underway again in 2000. 

Future research needs identified in conducting this study include (1) incorporating improved truck origindestination data by commodity from the Commodity Flow Survey and other sources into the ITIC freight

diversion model; 

(2) improving other essential logistics data in the ITIC model; (3) improving our understanding of

relationships between TS&W variables and truck safety risks; (4) examining the potential for new

technology to reduce adverse safety and operational characteristics of current vehicles; (5) improving

information on impacts of heavier vehicles on bridges and strategies to mitigate those impacts; and (6)

improving our understanding of ways to reduce traffic conflicts between large trucks and the rest of the

traffic stream.

Work to address some of these research issues can begin immediately, but even after the research has been

conducted, uncertainties about potential impacts of TS&W options will remain. It is extremely difficult to

accurately predict how the market might respond over time to changes in allowable TS&W limits or how

States might respond, particularly if longer multitrailer vehicles were to be an option. However, many

impacts are interrelated; if one impact is known others can more readily be estimated. 

While future research may allow more informed decisions about impacts of various TS&W policy options,

many TS&W decisions are fundamentally political and involve tradeoffs among equally worthy goals.
40

Conclusions 

State Perspectives

Significant productivity benefits were estimated for each illustrative scenario that allowed heavier vehicle

weights, but these benefits were derived primarily from the use of LCVs, even for 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. Even if Federal law did not require States to allow larger or heavier vehicles, some States

fear that if neighboring States allow LCVs, they will face irresistible pressure to also allow LCVs to keep

their businesses competitive. Federal TS&W limits thus act as buffers which protect States from industry

pressure to raise their TS&W limits.

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, that LCVs improve productivity, that

LCVs can operate without staging areas or interchange improvements, and that current grandfather laws

often result in LCVs having to operate off the Interstate System rather than on the safer Interstate

Highways.

Still other States would like increases in gross weights allowed for six-axle tractor-semitrailers and single unit

trucks like 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 effects of allowing only those shorter

vehicles. In general, allowing such vehicles at the weights analyzed in this study would not be expected to

cause additional pavement damage on Interstate Highways, nor would they increase costs to improve

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. Many bridges would have to be replaced, strengthened, or posted to

prevent vehicles operating at the proposed weight limits.

Truck Size and Weight Trends

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. Several States have been granted legislative

exceptions to Federal gross weight or axle-weight limits, including four States that received such exemptions

in TEA-21. States are granting increasing numbers of oversize and overweight permits,
41

especially for international containers, but also for many other commodities. In many cases such permits

allow unlimited use over a year’s period

While Federal laws have constrained increases in vehicle weights, the cubic capacity of vehicles has been

increasing, primarily as the result of increasing trailer lengths. For example, at the time of the last

Departmental report on TS&W limits in 1981, the standard trailer length was 45 feet with 48-foot

semitrailers being used in increasing numbers. Fifty-three foot long semitrailers are becoming a standard for

many carriers, and some States allow trailers up to 60 feet in length. Average operating weights of tractorsemitrailers 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. 

There are several implications of these ad hoc trends that are occurring while basic Federal TS&W limits

remain unchanged. With the increasing weights being allowed under permit, pavements and bridges will

deteriorate faster. 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. As trailer lengths have moved beyond 53 feet in some States, however,

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 U.S. will cause even greater pressures to

increase weight limits, especially in major trade corridors in this country. 

One scenario evaluated in this study, the Uniformity Scenario, would have virtually eliminated variations in

State TS&W limits, but little sentiment to roll back Federal TS&W limits to the extent assumed in this

scenario was expressed in docket comments. The H.R. 551 Scenario would phase out trailers longer than

53-feet and freeze weight limits on the National Highway System, but would retain existing grandfather and

other legislative exemptions to the basic Federal weight laws.

Cost Recovery

Cost recovery is an issue that several States mentioned in comments to the docket, and 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 perhaps pavement costs could increase. Some States

capture a large share of the additional infrastructure costs associated with operations of oversize and

overweight vehicles through permit fees, but other States charge fees that cover little more than costs to

administer the permit program. At the Federal level, there is no mechanism for capturing added costs 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. 

Next Steps

The TRB has a study underway of Federal TS&W regulations as requested in TEA-21. That study will

consider whether changes in Federal TS&W limits are advisable and evaluate how changes might affect the

economy, the environment, safety, and services to communities.
42

The Department 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 truck size and weight

policy workshop to discuss specific improvements that can be made in data and analytical methods used in

assessing impacts of truck size and weight policy options. The workshop also was intended to provide

solicit perspectives from a variety of stakeholders on future directions for Federal truck size and weight

policy. 

The Department will be prepared to update this TS&W study before the next surface transportation

reauthorization using updated data and analytical tools and building on other on-going research by TRB, the

National Cooperative Highway Research Program and other institutions. In the meantime, if requested by

Congress, the Department is prepared to examine additional TS&W options that may be of interest. An

analysis is already underway of a “Western Uniformity Scenario” as requested by the Western Governors

Association. 

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 cause little serious impacts to

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

potential costs, 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

productivity, safety, and international competitiveness.
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Notes to Table 2

 (1) "Routine" Permit Gross Vehicle Weight: the first number (left) is the highest weight a five-axle unit

can gross before special (other than routine) review and analysis of an individual movement is

required. The second number (right) is the highest gross weight any unit with sufficient axles can

gross before special review is required.

 (2) State rules allow the more restrictive of the Federal Bridge Formula or the sum of axle weight

limits. The five-axle "routine" permit value is estimated using a truck tractor-semitrailer with a 65-

foot outer bridge (based on a 48-foot semitrailer).

 (3) The five-axle "routine" permit value is estimated using a truck tractor-semitrailer with two tandem

axles at 47,250 pounds each and a 12,000 pound steering axle.

 (4) Estimate based on State weight table values for a' tandem drive axle at 46,200 pounds, a rear

tandem at the 60,000 pound maximum, and a 12,500 pound steering axle.

 (5) Maximum based on the number of axles in the combination.

 (6) Federal bridge formula applies if gross vehicle weight exceeds 73,280 pounds.

 (7) If gross vehicle weight is less than 73,280 pounds, the tandem axle maximum is 40,680 pounds.

 (8) On class III and non-designated highways the maximum is 73,280 pounds.

 (9) On non-designated highways the single axle maximum is 18,000 pounds, the tandem axle maximum

is 32,000 pounds, and the bridge formula does not apply.

(10) On the Indiana Toll Road the single axle maximum is 22,400 pounds, the tandem axle maximum is

36,000 pounds, and the maximum practical gross is 90,000 pounds.

(11) The maximum gross weight on class AA highways is 62,000 pounds, and on class A highways,

44,000 pounds.

(12) Six or seven-axle combinations are allowed 83,400 pounds on the Interstate System, and 88,000

pounds on other State highways.

(13) A three axle tractor hauling a tri-axle semitrailer has a maximum gross vehicle weight of 90,000

pounds.

(14) If the gross vehicle weight is less than 73,280 pounds, the single axle maximum is 22,000 pounds.

(15) If the gross vehicle weight is 73,000 pounds or less, the single axle maximum is 22,400 pounds, and

the tandem axle maximum 36,000 pounds.
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(16) Federal axle, gross and bridge formula limits apply to five-axle combinations if the gross vehicle

weight is 80,000 pounds or less. For other vehicles and gross vehicle weights over 80,000 pounds

other limits apply. State law sets axle weight controls which allow vehicles of legal overall length to

gross a maximum of 164,000 pounds.

(17) Most city, county and township roads are considered "9-Ton Routes" with a maximum gross

vehicle of 73,280 pounds.

(18) On highways other than Interstate, Primary, or other designated, the single axle maximum is 18,000

pounds, the tandem axle maximum 32,000 pounds, the bridge formula is modified, and the gross

vehicle weight maximum is 73,280 pounds.

(19) The maximum is directly controlled by the Federal bridge formula. Given the State's length laws,

the maximum practical gross is 129,000 pounds.

(20) The five-axle "routine" permit value is estimated using a truck tractor-semitrailer with a 12,500

pound steering axle, a 47,250 pound drive tandem (5-foot spacing from State weight table), and a

50,400 pound spread tandem (8-foot spacing from the State weight table).

(21) A determination is made on a case-by-case basis.

(22) All "routine" permit values are calculated using 10-inch wide tires and a maximum 800 pounds/inch

of tire width loading value.

(23) The five-axle "routine" permit value is estimated using a truck tractor-semitrailer with two 46,000

pound tandem axles and a 12,000 pound steering axle.

(24) If the gross vehicle weight is less than 71,000 pounds, the single axle maximum is 22,400 pounds,

the tandem axle maximum 36,000 pounds, and a modified bridge formula applies.

(25) If the gross vehicle weight is 73,280 pounds or less, the single axle maximum is 22,400 pounds, the

tandem axle maximum 36,000 pounds, and the bridge formula does not apply.

(26) If the gross vehicle weight is 75,185 pounds or less, the tandem axle maximum is 35,200 pounds,

and the bridge formula does not apply.

(27) The five-axle "routine" permit value is estimated using a truck tractor-semitrailer with two 52,000

pound tandem axles and a 12,000 pound steering axle.

(28) The five-axle "routine" permit value is estimated using a truck tractor-semitrailer with a 13,000

pound steering axle, a 45,000 pound drive tandem, and a 48,125 pound spread tandem. Both

tandem weight values are from the State weight chart.

(29) The five-axle "routine" permit value is estimated using a truck tractor-semitrailer with two 48,000

pound tandem axles and a 12,000 pound steering axle.
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(30) The maximum gross vehicle weight on non-designated State highways is 73,500 pounds, and on

county roads 65,000 pounds.