Final Report - November 2003
Introduction
Approximately 5,000 people are killed annually in crashes involving large trucks. Although less than 20% of the fatalities in such crashes are occupants of the truck, the truck occupant is often killed in situations that may be preventable had the occupant been wearing a safety belt.
The National Highway Traffic Safety Administration (NHTSA) has conducted a number of safety belt studies; however they have been limited to only automobile and light truck occupants. The "Safety Belt Usage by Commercial Motor Vehicle (CMV) Drivers" study was conducted by The Center for Applied Research, Inc. and Westat for the Federal Motor Carrier Safety Administration (FMCSA). The intent of this study was to design and implement a nationally representative sample survey of safety belt usage among CMV drivers and, based on the data collected, to produce estimates of safety belt usage rates for this segment of the driving population. The FMCSA will use the data to determine how best to allocate its resources to achieve its goal of 1.65 fatalities per 100 million truck vehicle miles traveled by 2008, and to increase safety belt use among commercial vehicle drivers.
Sampling Methodology
A three-stage sampling procedure was used in this study, conducted in 2002. In the first stage, a sample of twelve states was selected, using a "probability proportional to size" systematic sampling procedure, with State truck vehicle miles traveled (VMT) as the size measure.
In the second sampling stage, counties in each selected State were combined to form groupings that had a minimum of 300 limited-access highway miles. Once these county groups were formed, one county group was randomly selected from each selected State.
In the third sampling stage, observers visited each sampled county group, and selected locations, using county and city maps, where the safety belt usage of commercial trucks could be observed. These sites were selected on convenience rather than random sampling. When a potential observation site was identified, its truck volume was measured. If the site had at least two commercial vehicles passing through it within a five-minute interval, it was used for data collection. There were a total of 117 observation sites used in the study. Most of the sampled States had at least 10 sites.
Shoulder belt usage in vehicles was observed at each selected site. Observations were made over a two-day period in each of the 12 States. A minimum of 10 hours was spent collecting data in each State. Of the vehicles observed in the study, roughly 10% were classified as Class 7 (26,001 to 33,000 lbs.), and the remaining 90% were classified as Class 8 (33,001 lbs. and over), based on vehicle configuration and number of tires. Vehicles in Classes 1 through 6, with gross-vehicle-weights of less than 26,000 lbs., were not observed.
Results
A total of 3,909 trucks were observed. The overall safety belt usage rate for commercial vehicles observed in the study was 48%, with a standard error of 1.4% (standard errors were generated using the WesVar software program developed by Westat, Inc.). Based on this standard error estimate a 95 percent confidence interval for the usage rate ranges between 45 and 51 percent. According to statistical theory, if the study were to be replicated multiple times, 95 percent of the confidence intervals obtained from the replications would contain the true value for the safety belt usage rate in the population at large. Class 7 trucks had a safety belt usage rate of 54%. Class 8 trucks had a usage rate of 47
The usage rate for those units where the vehicle's tractor was identified as a major regional or national fleet was estimated to be 55%. For trucks that were either independent or part of local fleets, the usage rate was estimated to be 44%.
Safety belt rates were also estimated by commercial vehicle type, and by the presence of hazardous materials (HAZMAT). The vehicle type with the highest usage rate was found to be the single tanker (61%), and the vehicle type with the lowest usage rate was found to be the single-trailer dump truck (26%). Drivers of trucks pulling trailers placarded for HAZMAT were found to have the highest safety belt usage rate of all of the various categories observed (67%).
Safety belt rates by State should not be used from this study.
Study Limitations
Although the study has a number of limitations, as outlined below, it does provide a national estimate of safety belt use in Class 7 and Class 8 trucks. Since the survey design has a number of flaws, one cannot say with certainty that the observed belt use rate is as accurate or as nationally representative as it could have been if observed through a more rigorous survey. Even with these limitations, the survey does provide FMCSA with knowledge that safety belt use in heavy trucks needs to be improved.
The entire national fleet of vehicles regulated by FMCSA was not captured by this study. Specifically, only vehicles with gross vehicle weight ratings of more than 26,000 pounds appear to have been captured. The impact of not including commercial vehicles under 26,000 pounds in the study is unknown. Furthermore, vehicles in the study could not be weighed, and were classified as Class 7 or Class 8 vehicles, based on vehicle configuration and the number of tires on the vehicle. FMCSA has no way of knowing the reliability of such a protocol.
Several assumptions were made in the statistical methodology used in this study. As described above, observation sites were selected using convenience sampling. With this approach, the field investigator decides where to go in each selected county group to collect the required data. Inherent in this approach, is the assumption that the sampling locations selected by the investigator are representative of the county group as a whole, and do not differ dramatically in their characteristics from locations that would have been selected, had the sampling been performed randomly at this particular sampling stage.
The size measures used to select county groups within States varied from State to State, since VMT data were not available at the county level in all selected States. The estimation methodology used by the authors assumes that, in each State, the ratio of total size to county group size is equivalent to the ratio of total truck VMT to county group truck VMT (see Westat documentation in Appendix A). The weighting methodology further assumes that the average number of miles driven by a truck within a county group is the same for all selected county groups, thus allowing for some terms in the weighting formula to cancel-out.
Given the total sample size used (3,909 trucks), and the number of county groups selected in each selected State (one), the FMCSA does not believe the sample is sufficiently large to produce reliable estimates at the State level, for the 12 States used in the study. State level estimates presented in the document should be used only for purposes of comparison and should not necessarily be considered statistically reliable.