As part of the Moving Ahead for Progress in the 21st Century Act, the U.S. Department of Transportation has released the results of its Comprehensive Truck Size and Weight Limits Study. The conclusion: Data is too insufficient to recommend any type of regulatory change.
Throughout most of the 26-page report to Congress, the DOT explained in detail why the data available for the study did not provide definitive results or “yield a sound basis for any particular set of policy changes.” In fact, very little data was included.
In order to meet the requirements set by MAP-21, the study analyzed five technical areas:
- Highway safety and truck crash rates, vehicle performance (stability and control), and inspection and violation patterns;
- Shifts in goods movements among truck types and between modes;
- Pavement service life;
- Highway bridge performance;
- Truck size and weight enforcement programs.
Although the results are considered inconclusive, heavier and longer trucks were shown to be more cost efficient — but at the expense of safety.
Researchers looked at three configurations each for increased size and weight trucks. For heavier trucks, the following configurations were compared to the control group of five-axle tractor with 53-foot trailer vehicles weight 80,000 pounds:
- Five-axle tractor, 53-foot trailer weighing 88,000 pounds;
- Six-axle tractor, 53-foot trailer weighing 91,000 pounds;
- Six-axle tractor, 53-foot trailer weighing 97,000 pounds.
Vehicle miles traveled (VMT), logistics costs and enforcement program costs all decreased as the result of heavier trucks on the road. Enforcement effectiveness increased. However, limited crash data suggests possible increases in crash rates with the two heavier configurations. Although no national data was available, there was a 47 percent crash rate increase with 91,000-pound trucks in Washington state. For 97,000-pound trucks, crash rates increased in Idaho and Michigan by 99 percent and 400 percent, respectively.
With a weight increase of just 8,000 pounds, the study reveals longer stopping distances. Configurations over 80,000 pounds also had 18 percent more brake violations. Six-axle trucks did not exhibit any significant difference in stability and control when compared to the control group.
Truck size configurations used in the research include the following:
- Two 28 or 28.5-foot trailers weighing 71,700 pounds (control group);
- Twin 33s weighing 80,000 pounds;
- Triple 28s weighing 105,500 pounds;
- Triple 28s weighing 129,000 pounds.
As with heavier trucks, longer tractor-trailers resulted in a lower VMT, logistics costs and enforcement costs. Law enforcement effectiveness was also determined to be positive with longer trucks. No national data was available for crash rates, Idaho reported a 42 percent decrease in crashes with triple 28s weighing 105,500 pounds. There was a slight decrease (1 percent) on one roadway (Kansas Turnpike) for triple 28s weighing 129,000. Since twin 33s are not in common use, there was no available crash data of any kind for twin 33s.
All three longer configurations did not perform as well as the control vehicle in avoidance maneuvers, with twin 33s having a slightly longer stopping distance. “Amplification” of the third trailer’s response in both triple 28 configurations was greater than the control group, as was off-tracking. Not enough data was available to analyze violation rates for triple 28s, but twin 33s had higher inspection violations than standard five-axle single 53-foot trailers weighing 80,000 pounds.
Although the above results suggest that increasing size and weight will decrease costs and increase safety hazards, the study emphasized data limitations were too great to extrapolate any significant conclusion. The DOT explained that even with more time and money, meaningful results cannot be compiled without overcoming the myriad of limitations. Among the several recommendations for better studies, the DOT calls size and weight to be included in crash databases nationwide.
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