Comprehensive Truck Size and Weight Limits Study - Modal Shift Comparative Analysis Technical Report
Chapter 1: Introduction
Truck size and weight regulations in the United States represent a patchwork of Federal and State regulations that provides very little uniformity to shippers and carriers engaged in interstate commerce. Federal weight limits cover gross vehicle weight (GVW), permissible loads on single and tandem axles, and permissible loads on groups of axles intended to protect bridges. Current limits are 80,000 pounds GVW, 20,000 pounds on single axles, 34,000 pounds on tandem axles, and a complex formula (Bridge Formula B) that limits loads on groups of axles at different spacings.
Federal weight limits apply only to the Interstate System, although States cannot apply lower limits on the 200,000 mile National Network established in the Surface Transportation Assistance Act of 1982. Figure 1 is a map showing relationships between the Interstate System, the National Network, and the National Highway System.
Federal truck size and weight limits do not apply uniformly to Interstate Highways in all States, however. States that had higher weight limits when Federal weight limits were first enacted in 1956 were allowed to retain those higher limits under a "grandfather clause."  Furthermore, until 1991 States had the authority to reinterpret their grandfathered weight limits, which led to a gradual increase of weight limits in some States. The so-called ISTEA Freeze instituted in the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 froze the weights and trailer lengths of longer combination vehicles at levels determined to be in effect in a particular State in 1991. This still left a set of divergent weight limits applying to longer combination vehicles in States where those vehicles were allowed.
Each State has its own set of truck size and weight limits that apply to highways off the Interstate System. In some States, Federal and State limits correspond closely, while in others higher GVW or axle load limits are allowed off the Interstate System than are allowed under Federal law on the Interstate System. Further complicating State size and weight limits is the fact that some States have higher limits only for particular commodities, and some States have seasonal variations in weight limits as well.
An often-expressed concern is that higher State weight limits off the Interstate System cause truck traffic to use non-Interstate routes that may be less safe and may not have pavements and bridges designed to the same standards as those on the Interstate System. Studies have recently been conducted in Maine and Vermont to examine this issue in greater detail. Those studies examined the safety impacts of allowing some of the same vehicles as are being analyzed at a national level in this 2014 CTSW Study. Also, see the companion Volume II: Pavement Comparative Analysis document for additional information on this topic.
The most recent USDOT truck size and weight studies have focused primarily on the effects of allowing more widespread use of longer combination vehicles (LCV) (USDOT 2000, USDOT 2004). Those vehicles have substantially higher weights and higher cubic capacities than typical over-the-road tractor-semitrailers and could potentially draw freight from a wide variety of other types of vehicle as well.
Potential modal shifts to the tractor-semitrailers being examined in the current 2014 CTSW Study would be expected to be considerably less than diversion to the LCVs examined in the previous USDOT studies because fewer commodities would be able to benefit from the weight increases being examined. Most commodities shipped by truck in the United States fill the cubic capacity or floor area of the trailer before the vehicle’s maximum GVW is reached (in other words, they "cube out" before they "weigh out"). These commodities would realize no benefit from the increased tractor-semitrailer weights examined in this 2014 CTSW Study.
The purpose of this analysis is to estimate modal shifts associated with truck size and weight policy scenarios being analyzed by the U.S. Department of Transportation (USDOT) in the Comprehensive Truck Size and Weight Limits (2014 CTSW) Study called for in Section 32081 of the Moving Ahead for Progress in the 21st Century Act (MAP-21). Section 32081specifically requires an assessment of potential freight diversion associated with variations in truck size and weight limits. This report presents the estimated potential diversion associated with six truck size and weight policy options.
Within the context of this 2014 CTSW Study, freight diversion includes both the shift of freight traffic from railroads to trucks as well as the shift of truck traffic from one vehicle configuration to another and from one operating weight distribution that reflects Federal truck size and weight limits to another distribution reflecting higher weight limits.
The freight transportation system has grown increasingly complex as shippers and carriers continuously strive for greater efficiency in the face of high fuel costs, pressures to reduce greenhouse gas and other environmental emissions, difficulty recruiting drivers, and competition from a global marketplace. Even relationships between competing surface transportation modes have become more complex; trucking companies now are one of the railroads’ biggest customers. All of these factors present challenges to estimating how truck size and weight limits in excess of Federal maximums might affect nationwide freight transportation patterns.
Numerous truck size and weight policy studies have been conducted over the years at the Federal and State level. They have used a variety of analytical tools and data to estimate potential impacts of changes in truck size and weight limits on modal diversion. The desk scan conducted as part of this 2014 CTSW Study revealed that data and analytical tools have improved markedly over the past 20 years. This is especially true with respect to commodity flow data. Whereas the USDOT’s 2000 CTSW Study relied primarily on limited survey data collected at truck stops, the Federal Highway Administration’s (FHWA) Freight Analysis Framework (FAF) provides a much more robust picture of current commodity flows across the country.
Even with improved data there are many challenges in translating annual commodity flows between various origins and destinations (the exact locations of which are unknown) into annual truck volumes on different highway networks with weight distributions that match observed weight distributions. Vehicle weight distributions are particularly important since several of the truck size and weight options examined in this study only increase allowable weights with no increase in the cubic capacity of the vehicle. Thus only those shipments that reach the maximum payload of current vehicles before they fill the cubic capacity of the vehicle would be able to take advantage of the additional weight provided under several of the alternative federal truck size and weight limits examined in this 2014 CTSW Study.
Estimating potential adverse impacts on the railroads from truck size and weight limits greater than those outlined by Federal regulation is particularly important. While truck and rail are partners in some transportation markets, they are strong competitors in other markets. Considerable concern has been expressed, especially by short-line railroads, that increasing the productivity of trucks could have serious economic consequences not only on the railroads themselves, but on the communities they serve.
In addition to estimating modal shifts associated with variations in truck size and weight configurations, this task also requires estimates of how those shifts would affect energy consumption, environmental emissions, and highway traffic operations. The general state-of-the-art in energy, environmental, and traffic modeling has improved rapidly in recent years, and analyses in this 2014 CTSW Study reflect the latest understanding of factors that affect energy, environmental, and traffic impacts associated with the use of alternative truck configurations.
 The Federal government began regulating truck size and weight in 1956 when the National Interstate and Defense Highways Act (Public Law 84-627), establishing the Interstate Highway System, was enacted. A state wishing to allow trucks with sizes and weights greater than the Federal limits was permitted to establish “grandfather” rights by submitting requests for exemption to the FHWA. Claims that were not legally defensible were rejected. During the 1960s and 1970s, most grandfather issues related to interpreting State laws in effect in 1956 were addressed, and so most grandfather rights have been in place for many decades. See USDOT Comprehensive Truck Size and Weight Study, Volume 2, “Chapter 2: Truck Size and Weight Limits – Evolution and Context,” FHWA-PL-00-029 (Washington, DC: FHWA, 2000), p. II-9. Return to Footnote 3