ASET's Summary of the U.S. DOT's Truck Size and Weight Scenario Analysis

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Part III of the draft U.S. DOT Truck Size and Weight study was issued August 31, 2000 and reviewed a variety of scenarios for size and weight and their impact on safety, intermodal competition, infrastructure performance, economic productivity, traffic flow, environmental quality and energy consumption. These scenarios are expected to be used by policy makers and legislators in planning for the future use of our nation's highway system. The implications of the report's findings could be substantial for both motor carriers and shippers alike. Americans for Safe and Efficient Transportation (ASET) is primarily concerned with the study's review of scenarios for weight increases in existing 53 foot trailers. This summary is limited to those portions of the U.S. DOT draft report impacting weights. The complete report can be found on ASET’s web site at www.aset-safety.org.

BASE CASE

The base for projections of various size and weight scenarios is premised upon data available in 1994. The year 2000 is the scenario analysis year for which the projections are made. The base case uses all existing features of current law, including grandfathering of size and weights provided to many of the western states as of June 1, 1991. Current federal weight limits for interstate highways and bridges are used – 20,000-pound single-axle, 34,000 tandem-axle, 80,000 pound gross vehicle weight cap and federal bridge formula (FBF) continue.

WEIGHT SCENARIOS

The North American Trade Scenario focused on trade among North American trading partners (the U.S., Canada, and Mexico) by determining how trade could be facilitated by allowing the operation of six-axle tractor-semitrailer combinations at 97,000 pounds. Currently, the six-axle tractor semi-trailer configuration is widely used in both Canada and Mexico. To accomplish the 97,000 pound scenario a tridem weight limit of 51,000 pounds was tested. For a six-axle semitrailer combination, 12,000 pounds is assumed for the steering axle and 34,000 pounds on the drive tandem.

FREIGHT BISTRIBUTION

Allowing higher weights for a variety of current truck configurations, including the 97,000 pound scenario is anticipated to result in an overall reduction in heavy commercial truck vehicle miles traveled (VMT) of 11 percent in the year 2000. Five-axle tractor semitrailer VMT declines by 70 percent, decreasing from 83,895 million miles to 24,997 million miles. Shifting the freight to a six-axle tractor semitrailer results in only a 3 percent increase in VMT for these vehicles, going from 6,049 million miles to 6,246 million miles.

PAVEMENT

Savings in pavement restoration costs were largest under the North American Trade Scenario. An 11 percent decrease in VMT resulted in pavement restoration costs over 20 years of $2,447 million for a 97,000 pound vehicle scenario. The 70 percent reduction in VMT also results in predictions of much lower load equivalency factors (LEF). LEF’s ratings are essentially multipliers of the pavement wear of an 18,000 pound single-axle load. Accordingly, the LEF's for a 97,000 pound semitrailer load with a 51,000 pound tridem are 4.1 (front-axle), 8.4 (rear-axle), and 9.2 (tridem axle) for a six-axle semitrailer combination, versus 5.7, 9.3, and 10.3 for the five-axle semitrailer combination. (For more details see Chapter 5 of Volume III of the TS&W Study.)

BRIDGES

Impact of the use of a 51,000 pound tridem-axle are estimated at an additional $65 billion for capital costs and $264 billion for user delay costs. These costs can be compared to a current base of $154 billion in capital costs and $175 billion in user delay costs. However, the study freely admits that many of the bridges on the National Highway System are structurally and/or functionally deficient and will need to be repaired replaced in the near future.

SAFETY

US DOT acknowledges that VMT and safety are tied closely together. In other words, a change from a 80,000 pound five-axle semitrailer to a 97,000 pound six-axle semitrailer would result in fewer accidents (approximately 11% fewer if the VMT model is maintained throughout the study). Heavier vehicles are not inherently more prone to instability or roll-over, rather payload distribution is the most critical factor in controlling rollovers. Driver operational characteristics such as travel speed around curves, the “tightness” of the curve, etc… are also important factors.

Braking performance is admitted to not be particularly influenced by changes in vehicle sizes and weights, assuming that the required number of axles and brakes are added as the vehicles weight increases and the vehicle's brakes are well-maintained and functional. The requirement that antilock braking systems (ABS) be fitted to all new tractors and trailers is expected to enhance vehicle stability and control. Some incremental diminishments can be expected as truck weights are increased, but the greater concern in braking ability relates to longer combination vehicles.

ENVIRONMENTAL EFFECTS

A six percent decrease in fuel use would be achieved by transition to the 97,000 pound weight limit. Truck VMT in urban areas would decrease by more than 5 billion miles, resulting in a substantial decrease in air pollution control costs for these areas. Noise costs increase marginally due to the number of additional tires (approximately 15%) used and the consequence of larger loads on increased engine noise.

SHIPPER COSTS

Shippers converting to the use of six-axle 97,000 pound semitrailers would experience significant transportation savings. Truck shippers changing to higher GVW’s would save $13,277 million per year. Rail shippers changing from rail to truck would save $1,233 million per year.

* The U.S. DOT TS&W Study’s North American Trade scenario contemplates diversion of freight from five-axle trucks at 80,000 pounds to both six-axle trucks at 97,000 pounds (the ASET proposal) and a “new” twin 33-foot eight-axle double-trailer combination at 124,000 pounds. The eight-axle twin trailer is not currently used in the U.S., nor would ASET’s legislative proposal extend to the larger double-trailer combination. The U.S. DOT is unable to provide separate statistics on presumed diversion for single-trailers only.