Road Management & Engineering Journal
Road Management & Engineering Journal
Copyright © 1999 by TranSafety, Inc.
February 1999
TranSafety, Inc.
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Indiana Study Shows Double-Trailer Vehicles Not Overinvolved in Crashes

Large truck crashes, especially those involving tractor-trailers, are a significant safety concern on the nation's roadways. Tractor-trailers have higher fatal crash rates than either single unit trucks or passenger vehicles. In 1995, Federal Highway Administration (FHWA) statistics showed 12 percent of all people killed in traffic crashes were involved in large truck crashes, and about 75 percent of those deaths occurred in crashes involving tractor-trailers. Western doubles or twin trailers (a tractor pulling two trailers) are common truck configurations permitted on the interstate highways of every state. However, one particular type of multiple-vehicle trailer known as a longer combination vehicle (LCV) is permitted in only 20 states because of safety concerns. And most of the states that allow them limit their type, size, and operating routes.

Many researchers have studied crashes involving various tractor-trailer combinations, and they have drawn different conclusions about their crash risk. Contributing to the research base, a recent study "determine[d] whether multiple-trailer combination vehicles were overinvolved in crashes on interstate highways in Indiana relative to their proportion in the traffic stream." Elisa R. Braver, Paul L. Zador, Denise Thum, Eric L. Mitter, Herbert M. Baum and Frank J. Vilardo reported the study's results in "Tractor-Trailer Crashes in Indiana: A Case-Control Study of the Role of Truck Configuration" (Accident Analysis and Prevention, Vol. 29, No. 1, 1997). They found "no overall increase in crash risk . . . among tractors pulling two trailers relative to tractors pulling one trailer," but concluded "the crash risk of double-trailer vehicles might be greater under less favorable operating conditions and road environments." Double-trailer vehicles did show an increased crash risk on snow, ice, and slush. Because they lacked some control data, the authors could not draw definitive conclusions on the "intrinsic crash risk of multiple versus single-trailer vehicles." They suggested a need for more research on the relationship between truck configuration and other factors.


The Indiana study "was intended to be a prospective matched case-control study that would permit evaluation of the crash experience of double-trailer and single-trailer vehicles during 1989-1991 on interstate highways." Indiana University's Transportation Research Center (TRC) collected the data after state police notified the center (within 24 hours) of all tractor-trailer crashes (a tractor pulling one or two trailers). The state police failed to notify TRC of about half the eligible cases, but these cases were included in the study anyway, minus control observations--which the authors felt limited the study to a degree. The initial and added crash cases totaled 2033. As suggested by the Transportation Research Board (1989), the study also recorded "the configuration for all tractor-trailers passing by crash sites for a 30-minute period 1 to 4 weeks after the crash." The authors believed "obtaining the controls from the crash sites provided more accurate estimates of the double-trailer vehicles at risk for crashing than simply doing random traffic observations." Control cases totaled 62,919, but only five of these were on snowy/icy/slushy roads.

The study included data on tractor-trailer ownership (fleet size and type of carrier) and truck drivers (birth date, state license, and possible citation following the crash). Authors used a Standardized Crash Ratio (SCR) "to summarize the crash risk of double-trailer vehicles compared to single-trailer vehicles." SCRs above 100 indicated "crash overinvolvement" and those less than 100 indicated "crash underinvolvement."

Authors of the Indiana study compared their results with a similar Washington State case-control study by Stein and Jones (1988), which concluded:

"Double-trailer vehicles, including both western doubles and LCVs, had a significant threefold increased risk of crash involvement compared to single-trailer vehicles, even after adjusting for other variables significantly affecting crash risk, including empty or loaded travel, driver age, hours driving, type of carrier, and whether the carrier operated intrastate or interstate."


Results of this Indiana study showed that "among all crashes combined, no excess involvement of double-trailer vehicles in crashes was observed relative to what was expected; double-trailer vehicles actually were underinvolved, albeit nonsignificantly, in crashes." Specifically, "[s]ignificant underinvolvement of double-trailer vehicles was observed in multiple vehicle crashes (SCR=74), crashes occurring on dry road surfaces (SCR=61), and crashes occurring on wet (excluding snow, ice, and slush) road surfaces (SCR=54)." In addition, "[d]ouble-trailer vehicles were nonsignificantly underinvolved in crashes resulting in fatalities or serious injuries (SCR=66), crashes on curved roads (SCR=86), and crashes on roads with a grade (SCR=89)." However, close to half (45 percent) of double-trailer crashes occurred on snow, ice, or slush-covered roads, compared with 21 percent of single-vehicle crashes.

In further comparing double- and single-trailer vehicles in crashes, the study showed that for double-trailers the mean fleet size "was significantly larger," the mean age of their drivers "was significantly higher," and they "were significantly less likely to be owned by private carriers." Moreover, drivers of double vehicles "were significantly less likely to be given a traffic citation related to the crash" and "significantly more likely to be licensed in Indiana or nearby states."

Comparisons between double-trailer vehicles in the Indiana and Washington studies revealed "dissimilarities" in fleet ownership and size, driver age, and citations issued. Notably, "about 10% of double-trailer vehicles in crashes in the state of Washington study were from fleets of 250 or more in contrast to 99% of double-trailer vehicles in crashes in Indiana." In addition, "the median and mean ages of drivers of double-trailer and single-trailer vehicles in Washington crashes were much closer together than in Indiana." And in Washington, "drivers of double-trailer vehicles in crashes . . . had about the same likelihood as drivers of single-trailer vehicles in crashes" of being given a traffic citation. In Indiana, by contrast, "single-trailer drivers were more than three times as likely to be cited for the crash as Indiana double-trailer drivers."


The findings of the Indiana study "were similar to other studies that reported no strong or consistent increase in overall crash risk of multiple-trailer vehicles on interstate highways." However, the authors noted "that the relatively safe operation of doubles in Indiana may have been attributable to their driver and carrier characteristics rather than their inherent properties." In other studies, "young age (below age 27) and small firms have been shown to be associated with higher crash rates." As a result, "the double-trailer drivers in Indiana may have been a population at lower risk of crash involvement than the single-trailer drivers." The authors emphasized that "the good crash experience of double-trailer operations in Indiana may not necessarily be true of multiple-trailer operations in other states."

In addition to driver population and carrier characteristics, authors of the Indiana study felt the significant differences in crash risk between vehicles in their study and in the Washington State study might be attributed to the mountainous interstate highways in Washington, where curves and steep grades could increase crash risk.

The findings of both the Indiana and Washington studies showing higher crash rates for double-trailers on snow, ice, or slush were in keeping with other studies that found "weather and road surface condition are important determinants of whether trucks and passenger vehicles crash." The authors concluded this heightened crash involvement "may reflect intrinsic vehicle handling characteristics," but voiced concern regarding the apparent increase in hazard for tractors pulling trailers on winter-weather roads and believed "some state restrictions on truck travel on roads with snow, ice, or slush might be appropriate, especially for tractor-trailer rigs without antilock brake systems." In tests, antilock brakes have proven beneficial, but "no studies have been done of the effect of antilock brakes on real-world crashes of combination vehicles." Antilock brakes have been "mandated for newly manufactured tractors starting in 1997 and trailers in 1998," and "should improve stability of both multiple-trailer and single-trailer vehicles on snow and ice-covered roads."

A "major limitation" of the study was the lack of control data "for variables that might confound the relationship between truck configuration and crash risk, including fleet size, type of truck ownership, condition of vehicle, and driver characteristics such as age." Cost prohibited collecting this control data. The authors emphasized that "because truck configuration was highly associated with driver age and work operation attributes for trucks in crashes, the absence of control data on these potential confounders limits the conclusions on intrinsic crash risk of multiple versus single-trailer vehicles that can be drawn from this study." They advised "more [nationwide] research . . . on the relationship between truck configuration, road surface condition, and crash risk" and emphasized careful control of "travel, driver, work, and vehicle variables."

Copyright © 1999 by TranSafety, Inc.

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