CRASH

CITIZENS FOR
RELIABLE AND
SAFE HIGHWAYS

OVERWEIGHT TRUCKS

ARE MORE DANGEROUS TRUCKS

American roads and highways are already extremely dangerous. From 1977 through 1988 more American men, women and children were killed in crashes on U.S. highways than died in all U.S. wars from the Revolutionary War through the Vietnam War.

Trucks cause a disproportionate part of that carnage. Each year about 4,000 Americans die and 100,000 are injured in truck crashes.

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MORE WEIGHT MEANS MORE CRASHES

Truck crash rate studies conclusively show that the fatal truck crash rate increases when heavier loads are put in trucks. The following graph shows how rapidly the death rate goes up as loaded trucks get heavier.1

1 K.L. Campbell et al., "Analysis of Accident Rates of Heavy-Duty Vehicles", University of Michigan Transportation Research Institute, Report No. UMTRI-88-17, Ann Arbor, April 1988, p. 65.

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The meaning of "fatal crash involvement rate" in the horizontal axis on the graph on the preceding page is explained in footnote 2.2

The following sections of this white paper sets forth the many reasons why heavier trucks crash more frequently than less heavy ones.

1. Rollover

Heavier trucks have more rollover crashes.

Because of their higher centers of gravity, trucks are more susceptible to overturning during steady turns or sudden obstacle avoidance maneuvers than are passenger cars.

The center of gravity is, however, not the only factor that influences susceptibility to rollover. Engineering research at the University of Michigan's Transportation Research Institute has shown "categorically" that the higher a truck's gross vehicle weight, the greater its susceptibility to rollover. The results of that engineering research is shown graphically on Appendix A to this paper.

2 The horizontal axis on the graph on the preceding page (the fatal crash involvement rate) represents whether a truck weight category is over or under-represented in fatal crashes relative to the fatal crashes of other weight categories. A fatal crash involvement rate of 1.0 for a truck weight category would mean that such category had a fatal crash rate per million miles travelled that is equal to the average for all truck weight categories. An involvement rate of 1.42 means that this category had a crash rate per million miles travelled that is 42% above the average. An involvement rate of 0.64 means that the crash rate for this category is 36% below the average.

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3 "Gross weight increases . . . categorically reduce the roll stability of the vehicles studied. ... Gross weight increases are seen to have a definite and consistently degrading influence on rollover threshold." (R.D. Ervin et al., "Influence of Size and Weight Variables on the Stability and Control Properties of Heavy Trucks," Final Report, Vol I, University of Michigan Transportation Research Institute, March 1983, pp. 79 and 82.) Note in the chart in the Appendix to this White Paper that a tractor semi-trailer with a 73,300 pound gross weight has a rollover threshold of .400 g's. The same tractor-semitrailer with at 92,000 pound gross weight has a rollover threshold of .275 g's, 30% worse than that of the 73,300 pound truck. Id. Figure 30, p. 80.) An arithmetic degradation in the rollover threshold causes an almost geometric increase in rollover accidents.

Crash rate studies lead to the same conclusion as do the engineering studies. Actual truck rollover crash rates increase as the gross vehicle weight increases.*

Indeed, the crash rate data studied together with University of Michigan Transportation Institute computer simulations show that the 1974 federal law which raised the maximum permitted gross vehicle weight from 73,000 pounds to 80,000 pounds resulted in a 44% increase in the rollover rate of fully-loaded trucks."

2. Runaway Trucks

Heavier trucks would cause more "runaway" crashes on downhill grades. Because of the vast number of hills and mountains in this country trucks must descend many steep downhill grades.

Increased truck weights causes truck brake temperatures to increase during downhill descents. This can lead to total brake failure and runaway crashes.

The crash data show that heavily loaded trucks are over-represented in runaway truck crashes relative to less heavily loaded ones." This means that the probabilities of runaway crashes on downhill runs will increase as truck gross weights increase.R

The probability of a runaway today (before a legal increase in gross vehicle weight) is already one runaway in every 5,000 downhill trips. Interviews of truck drivers

with mountain driving experience show that one out of four has lost his brakes during his driving career.

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R.D. Ervin et al., "Influence of Size and Weight Variables on the Stability and Control Properties of Heavy Trucks," Final Report, Vol. I, University of Michigan Transportation Research Institute, March 1983, pp. 82-84. See also K. L. Campbell et al., "Analysis of Accident Rates of Heavy-Duty Vehicles," University of Michigan Transportation Research Institute, Report No. UMTRI-88-17, Ann Arbor, April 1988.

S R.D. Ervin et al., supra note 5, p. 84.

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Transportation Research Board, National Research Council, Truck Weight Limits,Special Report 225, Washington, D.C., 1990, p. 119.

7 P.S. Fancher et al., "Retarders for Heavy Vehicles: Evaluation of Performance Characteristics and In-service Costs," Phase I, Technical Report, Highway Safety Research Institute, The University of Michigan, February, 1981, pp. 90-91.

Transportation Research Board, supra note 6, pp. 119-120.

More weight means longer stopping distances for trucks with out-of-adjustment brakes.

Unfortunately, truck brakes are not well-maintained in the United States. In 1992, twentyfour percent of the trucks stopped and inspected by the highway patrols and state police forces of our country had maintenance problems so severe that they were ordered off the road. The problem with most of those poorly maintained trucks was that their brakes or suspensions were unfit for further travel."1

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If a truck's brakes are not properly adjusted, the truck's minimum stopping distance will increase approximately in direct proportion to the increase in the truck's gross weight." If the 80,000 pound weight limit currently in force on Interstates is not made applicable to the entire National Highway System, many trucks will be substantially heavier than they would otherwise be and will have poorly adjusted brakes. Those trucks will travel substantially further after the drivers put on their brakes than they would have traveled had the 80,000 pound limit been applied.

Truck crash data show that heavier trucks lead to more crashes caused by those trucks' inability to stop in time. A University of Michigan study concluded that the rates of fatal crashes in which tractor-semitrailers rear-ended other vehicles were consistently higher for more heavily loaded trucks than for more lightly loaded trucks.13

Note that this data comes primarily from states that have 80,000 pound maximum weight laws. The University of Michigan study generally compared crash rates for trucks weighing 30,000 or 40,000 pounds against crash rates for trucks weighing 65,000 or 80,000 pounds.

b. Overbraking and Jackknifing

11 Federal Highway Administration, Office of Motor Carriers, Accomplishments and Effectiveness Annual 1992 Report, Attachment 111.

12 R.D. Ervin et al., "Influence of Size and Weight Variables on the Stability and Control Properties of Heavy Trucks," Final Report, Vol. I, University of Michigan Transportation Research Institute, March 1983, pp. 55 and 76.

13 P.S. Fancher et al., "Turner Truck Handling and Stability Properties Affecting Safety," Final Report, Volume I, University of Michigan Transportation Research Institute, 1989, p. 122.

A 120,000 pound weight limit can lead to more jackknife crashes when trucks travel empty.

Truck braking systems are designed to provide adequate braking capacity when the truck is fully loaded. 120,000 pound trucks must have brakes designed to stop 120,000 pound loads. Brakes designed for 120,000 pound trucks are more likely to apply too much torque to the tires when trucks are empty than brakes designed for 80,000 pound trucks. This frequently leads to jackknifing crashes."4

4. Steering Dangers

The steering sensitivity of tractor-semitrailers decreases sharply as the gross vehicle weight increases. 15

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Crash data show that the lower the steering sensitivity, the higher the fatal single vehicle truck crash rates." Thus, increases in truck weight will translate into more single vehicle fatal crashes due to poor steering sensitivity.

5. High Speed Offtracking

Offtracking means that the rear wheels of a trailer do not follow the front wheels of the tractor. High speed offtracking results when a tractor-semitrailer goes around a curve or a freeway exit ramp at or near highway speeds.

14 Transportation Research Board, National Research Council, Truck Weight Limits, Special Report 225, 1990, p. 111. Note that loaded trucks seldom jackknife. Jackknife is usually initiated by premature lockup of the drive axles of tractors hauling empty trailers. P.S. Fancher et al., "Vehicle Design Implications of the Turner Proposal," University of Michigan Transportation Research Institute, September 14, 1989, text accompanying Figure 8. Heavier trucks require more torque capacity than do lighter trucks. When the torque level applied to tires becomes too great, given the load on those tires, lockup and loss of control occurs. R.D. Ervin et al., supra note 4, p.52.

15 Fancher et al., 1989, supra note 9, pp. 92 and 96.