The results of this study indicate that implementation of the mandatory alcohol testing programs in 1995 is associated with a 23% reduction in alcohol involvement in fatal crashes by motor carrier drivers. The estimated safety benefit of the mandatory alcohol testing programs is consistent across age groups and between sexes. Moreover, implementation of these programs has reduced alcohol involvement by motor carrier drivers in daytime and nighttime fatal crashes to a similar degree.
Evidence for the effectiveness of the mandatory alcohol testing programs in reducing alcohol-related crashes is strengthened by the study design and statistical approach. Controlling for confounding factors, particularly unmeasured confounders, is a serious limitation inherent to all observational studies. This study took advantage of data on multivehicle crashes, in which motor carrier drivers were matched with the comparison drivers on tempo-spatial variables both measured (e.g., year, month, day of the week, time of day of the crash, geographic region, US state, location, road conditions, weather conditions) and unmeasured (e.g., socioeconomic environment, regulatory changes other than the mandatory alcohol testing program, and variations in law enforcement). The study design made it possible to substantially reduce the effect of confounding factors. Inclusion of the interaction term between driver type and presence/absence of the intervention in the statistical model enabled us to construct a series of linear contrasts based on the regression coefficients and to interpret the results from the logistic regression models with clarity.
Matching on tempo-spatial variables in multivehicle crash data provides considerable advantages for bias control. This feature is especially appealing in studies of fatal motor carrier crashes because 79% of these crashes are collisions between motor carriers and other vehicles. Although only 19% of fatal motor carrier crashes involved a single vehicle, they accounted for 66% of motor carrier driver fatalities. The estimated effect of mandatory alcohol testing programs on alcohol involvement in fatal crashes by motor carrier drivers appears to be robust. Including single-vehicle crashes and multi–motor-carrier crashes in the analysis attenuated the estimated effect size from −23% to −21%. Snowden et al. (9
) analyzed FARS data from 1988 to 2003 for large-truck drivers and all light-passenger-vehicle drivers and found that implementation of the mandatory alcohol testing programs was associated with a 15% reduction in the risk of alcohol involvement in fatal crashes by large-truck drivers. The modest difference in the estimated effect size between the Snowden et al. report and the present study is likely due in part to the different study designs. Whereas matching on tempo-spatial variables should enhance internal validity of our study results, the time-series approach by Snowden et al. might have the benefit of greater generalizability.
Our analysis also indicates that drivers who are 25–34 years of age, are male, or have a positive DWI history are at a significantly increased risk of alcohol involvement in fatal crashes. This finding, which is consistent with previous research in other driver population groups (28
), may help target high-risk motor carrier drivers for interventions to further reduce alcohol-related crashes.
Our study has several limitations. First, our analysis relied largely on multiply imputed alcohol data because alcohol testing was performed on only 40% of the study subjects. Although multiple imputation is superior to many other missing data methods (25
) and appears to mitigate information bias due to missing data, imputed BACs cannot completely substitute for actual alcohol testing results.
Second, this study did not account for other regulatory changes and interventions that might have differentially affected the likelihood of alcohol involvement in fatal crashes by motor carrier drivers and other drivers. Although the study design may have controlled for tempo-spatial variables with nondifferential effects on motor carrier drivers and other drivers, policy changes specifically targeted at 1 of the 2 groups may still threaten validity of the study results. For instance, after 1995, many states lowered the legal alcohol limit for operating a motor vehicle from 0.10 g/dL to 0.08 g/dL. This change in alcohol policy, however, should have far less impact on motor carrier drivers than on other drivers because the legal alcohol limit for motor carrier drivers has remained at 0.04 g/dL since implementation of the mandatory alcohol testing program in 1995.
Finally, our outcome variable, alcohol involvement in fatal crashes, does not directly measure the risk of alcohol-related fatal crashes. Although national statistics indicate that rates of fatal crashes involving large trucks decreased considerably during the study period (29
), it is unwise to interpret the estimated reduction in alcohol involvement in fatal crashes by motor carrier drivers as a reduction in alcohol-related crashes. Moreover, this study was limited to fatal crashes. Further research is warranted to determine the effectiveness of the mandatory alcohol testing program for motor carrier drivers in reducing alcohol involvement in nonfatal crashes.
Despite these limitations, this study provides compelling evidence that implementation of the mandatory alcohol testing programs has significantly reduced alcohol involvement in fatal motor carrier crashes. Mandatory alcohol testing programs for employees with safety-sensitive functions have been challenged in courts of law by employers on the basis of unnecessary costs and by unions on the grounds of unreasonable search in violation of the Fourth Amendment (17
). Free cross-border trade by motor carriers, a major objective of the North America Free Trade Agreement, has been hindered because of safety concerns. Among the differences in regulations for motor carriers is mandatory alcohol testing, which is required of drivers in the United States but not in Canada and Mexico. Whether Canada and Mexico should adopt the mandatory alcohol testing policy for motor carrier drivers remains an outstanding issue under trilateral negotiation. Results of our study should be valuable in advancing the discourse on mandatory alcohol testing programs.