Underriding Evidence

Jan. 1, 2001
Are underride guards needed for straight trucks? The National Highway Traffic Safety Administration said no five years ago. Now NHTSA-sponsored research

Are underride guards needed for straight trucks? The National Highway Traffic Safety Administration said no five years ago. Now NHTSA-sponsored research questions that conclusion.

IN 1996, the National Highway Traffic Safety Administration announced it would mandate underride guards on trailers, but it would not require guards on straight trucks.

The agency also reserved the right to change its mind.

In its final rule establishing FMVSS 223 and 224, NHTSA exempted straight trucks from the underride regulations. However, the agency said in the January 24, 1996, Federal Register that it "may supplement this action by initiating a separate rulemaking action to consider rear impact guards for single unit trucks."

NHTSA has chosen to look into the issue more closely, sponsoring research through its Center for National Truck Statistics. The results of that research are becoming more widely disseminated, most recently at the Society of Automotive Engineers International Truck & Bus Meeting & Exposition in Portland, Oregon, December 6.

Appearing as part of a panel on crashworthiness and occupant protection, Dr Daniel Blower concluded that "If underride guards are appropriate for semitrailers, it appears they are also appropriate for straight trucks."

Blower is an assistant research scientist for the Survey and Analysis Division for the University of Michigan Transportation Research Institute (UMTRI). He and Kenneth Campbell coauthored the report that was sponsored by NHTSA. The two also wrote a 2000 SAE technical paper on the subject that Blower presented at the International Truck & Bus Meeting.

Checking the Numbers The survey was based on the Trucks Involved in Fatal Accidents (TIFA) statistics, a subset of the Fatality Analysis Reporting System (FARS) file that NHTSA produces.

The researchers filled in details in otherwise sketchy accident reports by telephoning those involved with the accidents. These include the truck drivers, owners, fleet safety directors, and the police officers who investigated the accidents.

Concentrating on rear-end fatalities, the researchers only considered light-duty vehicles that crashed into trucks during the 1997-1998 data year. They wanted to know if the truck had a rear underride guard, whether the striking vehicle underrode the truck, and to what extent the vehicle underrode the truck.

Among the information collected were the amount of body overhang relative to the rear wheels of the truck, the amount of cargo overhang, the height of the truck body floor from the ground, whether the truck was equipped with an underride guard, the guard's width and its height from the ground, and any other equipment on the rear of the truck that extends below the floor of the truck body.

The Underride Mandate Underride guards have been mandatory on trailers since 1998. However, NHTSA decided not to require them on straight trucks when the agency issued its final rule on the issue - for several reasons. Among the facts NHTSA presented in its final rule:

- Most (73%) of those who died as a result of rear-end collisions struck trailers, not straight trucks.

- Single-unit trucks represented 72% of registered heavy vehicles.

- Based on the previous two items, the agency concluded that a trailer underride rule would affect 28% of heavy vehicles, but it would address 73% of the fatalities resulting from rear-end collisions.

- The National Truck Equipment Association opposed including straight trucks on the basis that the high number of special purpose vehicles would result in more engineering and higher costs (up to $3,000 for a custom-made guard).

NHTSA said "this category of vehicles should not be covered by the rule at this time." The agency added that it may be desirable to cover at least some single-unit trucks, but regulators said they did not have enough information on straight-truck accidents when the final rule was published.

The research project had two objectives, Blower said - to evaluate the parameters of rear underride protection in straight trucks and to estimate the size of rear underride problems in fatal accidents.

What They Found The data revisited some of the complexities NHTSA encountered when the agency considered the truck and trailer underride issue.

When light-duty vehicles crash into the rear of a truck or trailer, the chances of underride increase when the truck or trailer has a high floor height and when the rear wheels are positioned several feet ahead of the rear end. Blower's research revealed a wide range of floor height and body overhang.

Among the straight trucks that were involved in fatal accidents during 1997-1998, the distance of the cargo bed floor from the ground varied from less than 22 inches (in about 4% of the cases) to more than 48 inches 12% of the time. Most of the trucks, as expected, had floor heights ranging from 31 to 48 inches.

Body style and overhang also varied significantly. Almost 1% of the trucks involved in fatal accidents had no body overhang whatsoever, while another 1% had more than 120 inches of overhang.

Van bodies had the most overhang. Among the trucks involved in fatal accidents, the mean distance between the rear wheels and the rear of the van body was just over 60 inches. Refuse trucks also had more than 60 inches of overhang. The overhang for platform bodies and tanks exceeded 50 inches. Dump bodies had the least overhang - approximately 35 inches.

Equipment and Underride Trucks are involved in fatal accidents, regardless of the presence of underride guards or equipment mounted at the rear of the vehicle, the research indicates.

Researchers found 148 fatal collisions in 1997-1998 in which straight trucks were struck from the rear, including 15 in which it could not be determined if the truck had a guard or rear-mounted equipment. Of the remaining 133 cases, just over half had a guard, equipment, or (in 7% of the cases) both. Approximately 47% of the accidents involved a straight truck that had neither a guard nor rear-mounted equipment such as liftgates or salt spreaders.

"Overall, the survey results do not show that either underride guards nor mounted equipment had much effect on the amount of underride, although the sample sizes are small with only one year of data," Blower says.

According to survey results, the most severe underride occurred (all the way to the windshield of the light-duty vehicle) when the trucks had underride guards and equipment mounted below the cargo floor of the truck body. Of the 13 such accidents reported during the year, seven had underride to the windshield. Only two (15% of the total) did not underride the trucks with guards and equipment installed. By contrast, in 26% of the accidents involving trucks that had neither guards nor equipment, the light-duty vehicle did not underride the truck. Blower found 90 of these collisions occurred during the year.

"This result is counter to what would be expected, although it may be due to the small sample size and a host of other complicating factors," he says. "A fatality must occur for the crash to be included in FARS. It could be that many of the collisions are beyond the design limits of the guards, and so the guards have no effect."

Blower cites research conducted by the University of Indiana that concludes that the presence of underride guards would have made little difference in seven of 11 rear underride crashes in the data studied.

About the Author

Bruce Sauer | Editor

Bruce Sauer has been writing about the truck trailer, truck body and truck equipment industries since joining Trailer/Body Builders as an associate editor in 1974. During his career at Trailer/Body Builders, he has served as the magazine's managing editor and executive editor before being named editor of the magazine in 1999. He holds a Bachelor of Journalism degree from the University of Texas at Austin.