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Comprehensive Truck Size and Weight Limits Study - Highway Safety and Truck Crash Comparative Analysis Technical Report

Executive Summary


This report documents analyses conducted as part of the U.S. Department of Transportation (USDOT) 2014 Comprehensive Truck Size and Weight Limits Study (2014 CTSW Study). As required by Section 32801 of MAP-21 [Moving Ahead for Progress in the 21st Century Act (P.L. 112-141)], Volumes I and II of the 2014 CTSW Study have been designed to meet the following legislative requirements:

Subsection 32801 (a)(1): Analyze accident frequency and evaluate factors related to accident risk of vehicles to conduct a crash-based analyses, using data from states and limited data from fleets;

Subsection 32801 (a)(2): Evaluate the impacts to the infrastructure in each State including the cost and benefits of the impacts in dollars; the percentage of trucks operating in excess of the Federal size and weight limits; and the ability of each state to recover impact costs;

Subsection 32801 (a)(3): Evaluate the frequency of violations in excess of the Federal size and weight law and regulations, the cost of the enforcement of the law and regulations, and the effectiveness of the enforcement methods; Delivery of effective enforcement programs;

Subsection 32801 (a)(4): Assess the impacts that vehicles have on bridges, including the impacts resulting from the number of bridge loadings; and

Subsections 32801 (a)(5) and (6): Compare and contrast the potential safety and infrastructure impacts of the current Federal law and regulations regarding truck size and weight limits in relation to six-axle and other alternative configurations of tractor-trailers; and where available, safety records of foreign nations with truck size and weight limits and tractor-trailer configurations that differ from the Federal law and regulations. As part of this component of the study, estimate:

(A) the extent to which freight would likely be diverted from other surface transportation modes to principal arterial routes and National Highway System intermodal connectors if alternative truck configuration is allowed to operate and the effect that any such diversion would have on other modes of transportation;

(B) the effect that any such diversion would have on public safety, infrastructure, cost responsibilities, fuel efficiency, freight transportation costs, and the environment;

(C) the effect on the transportation network of the United States that allowing alternative truck configuration to operate would have; and

(D) the extent to which allowing alternative truck configuration to operate would result in an increase or decrease in the total number of trucks operating on principal arterial routes and National Highway System intermodal connectors.

To conduct the study, the USDOT, in conjunction with a group of independent stakeholders, identified six different vehicle configurations involving six-axle and other alternative configurations of tractor-trailer as specified in Subsection 32801 (a)(5), to assess the likely results of allowing widespread alternative truck configurations to operate on different highway networks. The six vehicle configurations were then used to develop the analytical scenarios for each of the five comparative analyses mandated by MAP-21. The use of these scenarios for each of the analyses in turn enabled the consistent comparison of analytical results for each of the six vehicle configurations identified for the overall study.

The results of this 2014 Comprehensive Truck Size and Weight Limits Study (2014 CTSW Study) are presented in a series of technical reports. These include:

Volume I: Comprehensive Truck Size and Weight Limits Study – Technical Summary Report. This document gives an overview of the legislation and the study project itself, provides background on the scenarios selected, explains the scope and general methodology used to obtain the results, and gives a summary of the findings.

Volume II: Comprehensive Truck Size and Weight Limits Study. This volume comprises a set of the five comparative assessment documents that meet the technical requirements of the legislation as noted:

  • Modal Shift Comparative Analysis (Subsections 32801 (a)(5) and (6)).
  • Pavement Comparative Analysis (Section 32801 (a)(2)).
  • Highway Safety and Truck Crash Comparative Analysis (Subsection 32801 (a)(1)).
  • Compliance Comparative Analysis (Subsection 32801 (a)3)).
  • Bridge Structure Comparative Analysis (Subsection 32801 (a)(4)).

Purpose of Safety Technical Report

The USDOT study team conducted the safety analysis described in this Volume II: Highway Safety and Truck Crash Comparative Analysis report to explore differences in safety risk and truck crash frequency between truck configurations currently operating at and below current Federal size and weight limits on the Nation's roadways (control vehicles) as compared with a set of six alternative vehicle configurations that would hypothetically operate above the established Federal limits. This report also compares crash frequency and severity for both the control vehicles and the six alternative configuration scenarios defined for the 2014 CTSW Study (see Table ES-1 for the truck configuration and weight scenarios analyzed).

The first three scenarios assess tractor semitrailers that are heavier than generally allowed under currently Federal law. Scenario 1 assesses a five-axle (3-S2) semitrailer operating at a GVW of 88,000 pound, while Scenarios 2 and 3 assess six-axle (3-S3) semitrailers operating at GVWs of 91,000 and 97,000 pounds, respectively. The control vehicle for these scenario vehicles is the five-axle semitrailer with a maximum GVW of 80,000 pounds. This is the most common vehicle configuration used in long-haul over-the-road operations and carries the same kinds of commodities expected to be carried in the scenario vehicles.

Scenarios 4, 5, and 6 examine vehicles that would serve primarily less-than-truckload (LTL) traffic that currently is carried predominantly in five-axle (3-S2) semitrailers and five-axle (2-S1-2) twin trailer combinations with 28 or 28.5-foot trailers that have a maximum GVW of 80,000 pounds. Scenario 4 examines a five-axle (2-S1-2) double trailer combination with 33-foot trailers with a maximum GVW of 80,000 pounds. Scenarios 5 and 6 examine triple-trailer combinations with 28.5-foot trailer lengths and maximum GVWs of 105,500 (2-S1-2-2) and 129,000 (3-S2-2-2) pounds, respectively. The five-axle twin trailer with 28.5-foot trailers (2-S1-2) is the control vehicle for Scenarios 4, 5, and 6 since it operates in much the same way as the scenario vehicles are expected to operate.

Table ES-1: Truck Configurations and Weight Scenarios Analyzed in the 2014 CTSW Study
Scenario Configuration Depiction of Vehicle # Trailers or Semi-trailers # Axles Gross Vehicle Weight
Roadway Networks
Control Single 5-axle vehicle tractor,53 foot semitrailer (3-S2) 5-axle vehicle tractor, 53 foot semitrailer (3-S2) 1 5 80,000 STAA 1 vehicle; has broad mobility rights on entire Interstate System and National Network including a significant portion of the NHS
1 5-axle vehicle tractor, 53 foot semitrailer (3-S2) 5-axle vehicle tractor, 53 foot semitrailer (3-S2) 1 5 88,000 Same as Above
2 6-axle vehicle tractor, 53 foot semitrailer (3-S3) 6-axle vehicle tractor, 53 foot semitrailer (3-S3) 1 6 91,000 Same as Above
3 6-axle vehicle tractor, 53 foot semitrailer (3-S3) 6-axle vehicle tractor, 53 foot semitrailer (3-S3) 1 6 97,000 Same as Above
Control Double Tractor plus two 28 or 28 ½ foot trailers (2-S1-2) Tractor plus two 28 or 28 ½ foot trailers (2-S1-2) 2 5 80,000 maximum allowable weight 71,700 actual weight used for analysis 2 Same as Above
4 Tractor plus twin 33 foot trailers (2-S1-2) Tractor plus twin 33 foot trailers (2-S1-2) 2 5 80,000 Same as Above
5 Tractor plus three 28 or 28 ½ foot trailers (2-S1-2-2) Tractor plus three 28 or 28 ½ foot trailers (2-S1-2-2) 3 7 105,500 74,500 mile roadway system made up of the Interstate System, approved routes in 17 Western States allowing triples under ISTEA Freeze and certain four-lane PAS roads on East Coast 3
6 Tractor plus three 28 or 28 ½ foot trailers (3-S2-2-2) Tractor plus three 28 or 28 ½ foot trailers (2-S1-2-2) 3 9 129,000 Same as Scenario 5 3

1 The STAA network is the National Network (NN) for the 3S-2 semitrailer (53 feet) with an 80,000-lb. maximum GVW and the 2-S1-2 semitrailer/trailer (28.5 feet) also with an 80,000 lbs. maximum GVW vehicles. The alternative truck configurations have the same access off the network as its control vehicle. Return to Footnote 1

2 The 80,000 pound weight reflects the applicable Federal gross vehicle weight limit; a 71,700 gross vehicle weight was used in the study based on empirical findings generated through an inspection of the weigh-in-motion data used in the study. Return to Footnote 2

3 The triple network is 74,454 miles, which includes the Interstate System, current Western States' triple network, and some four-lane highways (non-Interstate System) in the East. This network starts with the 2000 CTSW Study Triple Network and overlays the 2004 Western Uniformity Scenario Analysis, Triple Network in the Western States. There had been substantial stakeholder input on networks used in these previous USDOT studies and use of those provides a degree of consistency with the earlier studies. The triple configurations would have very limited access off this 74,454 mile network to reach terminals that are immediately adjacent to the triple network. It is assumed that the triple configurations would be used in LTL line-haul operations (terminal to terminal). The triple configurations would not have the same off network access as its control vehicle–2S-1-2, semitrailer/trailer (28.5 feet), 80,000 lbs. GVW. The 74,454 mile triple network includes: 23,993 mile network in the Western States (per the 2004 Western Uniformity Scenario Analysis, Triple Network), 50,461 miles in the Eastern States, and mileage in Western States that was not on the 2004 Western Uniformity Scenario Analysis, Triple Network but was in the 2000 CTSW Study, Triple Network (per the 2000 CTSW Study, Triple Network). Return to Footnote 3

At this point it is important to note that for the purposes of the study the control double has an approved GVW of 80,000 pounds; however, the GVW used for the control double in the study is 71,700 pounds based on data collected from weigh-in motion (WIM)-equipped weight and inspection facilities. This is a more accurate representation of actual vehicle weights than the STAA authorized GVW, and using the WIM-derived GVW also allows for a more accurate representation of the impacts generated through the six scenarios.

Approach and Methodology

The study team pursued three different approaches for examining the safety of the alternative configurations:

  1. Crash-based analyses,
  2. Analysis of vehicle stability and control, and
  3. Analysis of safety inspection and violations data.

This three-pronged approach reflects the study team's conviction that using multiple types of analyses would provide a richer understanding of the safety performance of each alternative configuration, particularly when faced with data uncertainties associated with the crash data. Each of the three approaches has its own advantages and limitations, but all are designed to provide a broad picture of the potential safety implications of changes in the limitations of truck size and weight.

USDOT believes that the safety assessment should be conducted as much as possible with crash data reflecting actual operations on U.S. roads. The crash-based analyses used police-reported crash data in State crash files, crash information collected by trucking companies, and truck exposure data; for example, the travel demand situation, which was developed from several sources. The USDOT study team placed significant emphasis on analyzing crash data because they are "real-world data," and thus are most valid in nature. Acknowledged experts within the transportation safety discipline have stated in several publications on the topic that analyzing crash data and data on injuries and fatalities is, in fact, the definition of "safety analysis" (AASHTO, 2010; TRB, 2011).


The USDOT study team encountered several challenges while developing the safety information necessary to produce nationally representative estimates of changes in truck safety that are associated with the six alternative configuration scenarios:

  • A lack of truck weight data for individual trucks in crash databases resulted in the State crash analyses comparing groups of control and alternative scenario trucks operating within State-specified maximum allowable GVW limits. As a result, the study team completed its comparison based on the number of axles on the vehicle rather than a comparison of vehicles at specific weights.
  • Limitations in Annual Average Daily Traffic (AADT) and weigh-in-motion (WIM) data restricted the crash analysis to rural and urban Interstates.
  • The lack of data elements in most State crash databases that would identify the configuration of a truck (e.g., 3-S2) limited the State crash analysis and the development of crash estimates to one State for Scenarios 2, 5, and 6 and two States for Scenario 3. Scenario 1 could not be analyzed due to the lack of truck weight records in the crash data and Scenario 4 could not be analyzed since that alternative truck configuration does not currently operate in the United States.
  • Due to the limited number of States with suitable data, the analysis of crash rates cannot be extended to other States or be used to draw meaningful conclusions on a national basis. This Lack of weight data on State crash reports also made it impossible to complete a comparative assessment between trucks operating at and below current Federal size and weight limits and trucks that operate above those limits.
  • Vehicle weight information reported by the States in the Federal Motor Carrier Administration's (FMCSA) Motor Carrier Management Information System (MCMIS) is not consistently reported, affecting the team's ability to categorize vehicles appropriately for the study.

Each of these challenges and their implications are discussed in detail in the crash analysis sections of Chapter 2.


Additional information was obtained from States and fleets describing the exposure (VMT) of the alternative and control truck configurations. The exposure data from the States had to be supplemented with WIM data provided for the study by the Traffic Monitoring Team of Federal Highway Administration's (FHWA's) Office of Highway Policy Information. Some regression models relating crashes per mile to exposure were estimated; these results are reported in Section 2.3. Records of crashes from fleets were generally available, but the carriers did not consistently provide detailed route-level exposure data. As a result, only simplified analyses were undertaken with fleet data.

A set of vehicle stability and control analyses was designed to supplement the limited crash analysis performed in the Study. This analysis compared performance of control and alternative truck configurations in specific maneuvers. The maneuvers included low-speed off-tracking, high-speed off-tracking, straight line stopping distance, brake in a curve, and avoidance maneuver. Performance metrics included stopping distance, maximum path deviation, off-tracking, rearward amplification and lateral load transfer ratio.

A third approach was undertaken in the Study to fill out the highway safety analysis. Records on violations and citations contained in FMCSA's MCMIS were inspected so as to determine the violation and citation rates for different truck configurations. As noted in the Limitations discussion, the gross combination weight field in MCMIS contains various vehicle weight values and so this aspect of the analysis was also very limited.


The analyses indicate that the safety implications of the alternative truck configurations vary across the array of vehicles examined. In general, for Scenarios 2 and 3, the six-axle configurations have higher crash rates than the five-axle tractor-semitrailer control configurations studied in the three States that fit the selection criteria. This is particularly evident in the two study States where six-axle trucks could run at weights close to the 97,000-lb. six-axle alternative truck configuration. Similar findings with respect to inspections and violations were observed. The six-axle configuration had higher violations, OOS rates, and brake-related violations per inspection when compared to the control group (i.e., the five-axle tractor semitrailer configurations at 80,000 lbs.). This differed from the six-axle configuration findings in the vehicle stability and control analysis.

The vehicle control and simulation analyses showed very marginal differences between the control and alternative truck configuration for the set of maneuvers evaluated. The differences between the crash and vehicle control and simulations findings could result from the fact that while crash rates reflect actual operations with various drivers in a variety of traffic, roadway and environmental conditions, the simulation-based analyses addressed specific controlled conditions, not reflecting that same range of operators or operating conditions. It was not possible to determine in this study what factors led to these differences. Further exploration is needed.

The Scenarios 5 and 6 findings involving triple-trailer alternative truck configurations also differed between the crash and vehicle stability and control methods. While no differences between triple-trailer and twin-trailer configurations was seen in the Scenario 6 Kansas Turnpike data, the crash rate analyses for Idaho (Scenario 5) indicated the triple-trailer crash rates to be lower than the twin-trailer configuration's rates. The vehicle stability and control analyses showed very marginal differences between the control and alternative configurations for the set of maneuvers evaluated. The Level 1 inspection summary data for safety inspections and violations showed that triple-trailer configurations tend to have lower violation rates than twin-trailer configurations. However, this is based on a very small sample size, and as a consequence, more rigorous statistics could not be conducted to explore this further.

Finally, in the scenario analyses, recall that the crash rates used in all scenario analyses were based on either one or two States. The use of rates from this limited number of States clearly raises significant questions concerning whether estimates could be considered nationally representative. USDOT does not believe nationally representative estimates could be developed from the data.

A major finding of this overall effort is that crash-based studies of truck size and weight using U.S. data are very difficult to conduct successfully. This is particularly true if the studies are based on the primary data sources in existence today – State crash files, State roadway inventory data, State AADT data and additional data on VMT for specific truck configurations. Fleet-supplied and MCMIS data were also found wanting.

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