section 2.0 Policy Implications; section 2.7 New Approach Proposed by TRB Truck Weight Study

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Comprehensive Truck Size and Weight (TS&W) Study

Phase 1—Synthesis

Working Paper 3—Pavements and TS&W Regulations

2.0 Policy Implications

2.7 New Approach Proposed by TRB Truck Weight Study

TRB's Truck Weight Study also developed a new approach for regulating the weights of vehicles over 80,000 pounds. Under this approach, the maximum weight carried on any group of axles over 40 feet in length would be given by W in the following formula:

W = 1,000 ( 9 L / 16 + 72 )

where L is the length of the axle group in feet. Further, for vehicles with gross weights over 80,000 pounds, maximum axle weights would be limited as follows:

15,000 pounds for single axles

34,000 pounds for tractor drive tandem axles

30,000 pounds for other tandem axles.

The idea behind this new approach was to address some potentially negative pavement, safety, and productivity aspects of the current bridge formula:

Formula B provides a relatively modest incentive for operating trucks with more axles and consequently less pavement impact. According to the formula, adding an axle increases maximum weight by 4,000 to 6,000 pounds. An additional load-bearing axle on a tractor typically adds 2,700 pounds to empty weight, and an additional load-bearing axle on a trailer typically adds 1,500 pounds to empty weight. Hence, the added payload for an extra axle is less than 3,300 pounds for a tractor axle and less than 4,500 pounds for a trailer axle. Adding an axle generally increases operating costs for fuel and tires and increases costs for new tractors or trailers. For some truckers, the opportunity to carry 4,500 pounds (or less) of additional payload is an insufficient incentive to overcome these cost increases. Increasing the added payload allowed for an extra axle would encourage more truckers to adopt vehicles with more axles.

If the 80,000-pound limit were eliminated, five-axle doubles could operate under Formula B and current axle weight limits of up to 92,000 pound (assuming a practical maximum steering-axle weight of 12,000 pounds and 20,000 pounds on each of the other four axles). These vehicles perform very poorly in terms of pavement wear per ton of freight carried because they have single rather than tandem axles. In carrying high-density, weight-limited freight, five-axle doubles are less efficient than the LCVs with seven or more axles that currently operate under special permits in western states, so fiveaxle doubles carry little weight-limited traffic in those states. In eastern states with more restrictive length limits, however, elimination of the 80,000-pound limit with gross weights controlled instead by Formula B would cause some freight to shift from conventional five-axle tractor-semitrailers to five-axle doubles. This shift would adversely affect pavements.

Formula B provides little incentive to distribute loads evenly among axles. Adding an axle increases maximum permissible weight by 5,000 to 6,000 pounds, even if the axle itself carries no weight. This anomalous feature of the bridge formula can promote the use of non-load-bearing dummy axles. For example, a three-axle dump truck with a wheelbase of 16 feet can carry 48,000 pounds under Bridge Formula B; however, by adding a nonload-bearing dummy axle, this vehicle can operate at 52,500 pounds. Uneven axle weight distributions and the use of dummy axles can worsen pavement wear. For example, a 20,000-pound axle followed by a 10,000-pound axle does 70 percent more damage to pavements than two 15,000-pound axles. Uneven axle weight distribution and the use of dummy axles also degrade vehicle handling and performance, which may have adverse safety consequences.

Enforcement of the bridge formula can be complex and time consuming, because it involves measuring spacings between individual pairs of axles and applying the formula (usually by use of a table) to different axle groups. Many permanent weigh stations have stripes painted on the pavement to help enforcement officials estimate vehicle lengths. At roadside weight checks with portable scales, however, it is often not practical to test for bridge formula violations.

The new approach would have approximately the same impact on bridges as the current bridge formula, but would help meet the pavement, safety, and enforcement problems outlined above. On the negative side, TRB's Truck Weight Study noted that the equipment and loading practices of many truckers operating vehicles over 80,000 pounds under grandfather exemptions are designed to take advantage of the current federal axle limits. These truckers would be placed at a disadvantage by having to operate under two different sets of limits: current federal limits and the lower limits called for by the new approach. Further, the pavement-related problems with the current bridge formula noted above might be more simply addressed by prohibiting lift axles and limiting five-axle doubles to 80,000 pounds or less.

[continued on next page, Page 14, section Section 3.0; Knowledge Gaps and Research Needs ]

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