FIGURE 7-1 Five-axle tractor-semi vehicle-miles traveled by operating weight (cumulative percentage). SOURCE: FHWA (1997).

FIGURE 7-1 Five-axle tractor-semi vehicle-miles traveled by operating weight (cumulative percentage). SOURCE: FHWA (1997).

benefits of liberalized size and weight limits. In-depth interviews were conducted with seven companies as case studies. Two of the companies reported that because they carry low-density cargo, their trucks always reach volume capacity at a gross weight lower than the present limit. The other five companies at least occasionally load their trucks to the weight limit. These five were asked to report the percentage fuel consumption reduction they would expect if the gross weight limit were increased sufficiently to add 14,000 lb to cargo capacity. The median estimate of fuel consumption reduction was 23 percent. This prediction appears optimistic, since it could be attained only if all trucks began all their runs fully loaded to the weight limit and the heavier trucks had the same rate of fuel consumption per mile as trucks in present operations.

TABLE 7-1 Some Illustrative Projections of Fuel Consumption Savings

Study

Limit Change

Change in Truck Vehicle Miles Traveled (%)

Change in Fuel Consumption (%)

DOT (2000)

97,000-lb six-axle semis and 131,000-lb short doubles nationwide

−11

−6

DOT (2000)

148,000-lb turnpike doubles on interstates nationwide; 124,000-lb short doubles on most roads

−23

−13

TRB (1990a)

89,000-lb 6-axle semis and 96,000-lb short doubles nationwide

−3

−2

TRB (1990b)

110,000-lb short doubles nationwide where compatible with bridges

−3

−2

NOTE: Short doubles in the studies were twin 28- or 33-ft trailers; turnpike doubles were twin 53-ft trailers. The TRB studies did not estimate fuel savings; the values above are consistent with the studies’ traffic change estimates.

The impacts of past U.S. limit revisions have never been systematically monitored. In the United Kingdom the consequences of a recent change in the weight limit have been measured (McKinnon, 2005). In 2001, the U.K. weight limit was raised from 41 to 44 metric tons, allowing a 12 percent increase in maximum payload weight. Dimensional limits were not changed. Extrapolating the first two years of data on trends in truck travel and weight distributions, the study estimated that the eventual net effect would be a reduction of 170 million vehicle-kilometers/year by 2007, representing a 0.6 percent reduction in travel of freight vehicles. The percentage change in fuel consumption would be somewhat less. The estimate includes an allowance for road traffic generated by the reduced cost of truck transport. The impact has been small because a large fraction of freight vehicles do not operate at the weight limit. The characteristics of truck travel in the United Kingdom are different from those in the United States, but this example illustrates that the increase in fuel efficiency from a weight limit increase can be much less than the increase in maximum cargo capacity.

A 2002 TRB committee (TRB, 2002) reviewed the estimates of costs and benefits of revisions to size and weight limits in the past DOT, TRB, and Canadian studies. It concluded that liberalizing the regulations would reduce shippers’ freight transportation costs. Highway agencies costs for constructing and maintaining roads might increase or decrease, depending on the details of the regulatory changes and on how agencies changed their practices to accommodate the traffic changes; however, properly designed revisions to limits would yield freight cost savings exceeding any added extra infrastructure costs.



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