This study compared the sources of alcohol information available and the accuracy of police reports of alcohol involvement for drivers involved in a fatal motor carrier crash by merging population-based, cross-sectional data collected over a 24-year period. In this large cohort of drivers, there were significant differences between the motor carrier and non motor carrier drivers. While less than half (40%) of all drivers were chemically tested for alcohol, the testing rate was higher for motor carrier drivers compared to non motor carrier drivers. For the subgroup of drivers (25%) with a known BAC test result and a police-reported judgment about alcohol involvement, the sensitivity and specificity rates of the police reported alcohol data were reasonably high for both types of drivers (i.e., at least 83% and 91%, respectively). While the rates differed by other driver, vehicle, and crash characteristics as well, the patterns were similar by type of driver.
Although the sensitivity and specificity of the police-reported alcohol data were high, we do not recommend relying upon police-reported alcohol data because of the significant verification bias that was present in the evaluation of these data. Only 25% of drivers had both BAC testing and police-reported alcohol data. Verification bias occurs when the sensitivity and specificity of a test is not based upon the evaluation of all eligible subjects but rather a subgroup of subjects who receive either the index (police-reported judgment) or reference test (i.e., BAC test). In our study, we could only evaluate the sensitivity and specificity of police-reported alcohol data among drivers who also had a BAC test rather than among all drivers involved in a fatal motor carrier crash.
Previous studies that have evaluated the validity of police-reported assessments of alcohol involvement have also suffered from verification bias. For example, Williams and Wells (1993)
investigated alcohol involvement among passenger vehicle drivers who died in 23 states where chemical tests were performed in at least 80% of the cases. Their sensitivity and specificity rates of 92% and 85% were based only on the cases where BAC test results and police reported judgments were documented; they excluded 41% of cases in which the police did not report alcohol information. Similarly, Grossman et al. (1996)
reported a high sensitivity and specificity of police assessments for patients admitted to a Level 1 trauma center after being involved in a motor vehicle crash, however, the analysis was based on 22% of those initially hospitalized because of a motor vehicle injury. The remaining cases were not included because the crash records or alcohol information were missing.
Routine chemical testing is the most accurate way to estimate the role that alcohol plays in fatal crashes, to monitor trends, and to evaluate countermeasures such as laws or programs that are designed to reduce alcohol-impaired driving. The results of our study suggest that the conduct of BAC testing of drivers involved in a fatal crash needs to increase for drivers who die as well as those who survive. In this study, 68% of fatally injured drivers had a known BAC test result compared with 23% of surviving drivers. Approximately half of the states in our country have mandatory BAC testing of fatally injured drivers, 11 states authorize but do not require a test, and the remaining states have no law. In 2002, state testing rates of fatally injured drivers varied from 4% in the District of Columbia to 98% in Vermont (Hedlund, Ulmer, & Northrup, 2004
). States with the highest BAC testing rates of fatally injured drivers are those in which the state medical examiners have adopted the practice of conducting a BAC test on every traffic fatality so that they can determine whether alcohol was involved in the death (Hedlund et al., 2004
Among surviving drivers, only five states have mandatory BAC testing. The majority of states (i.e., 30) require that surviving drivers submit to a test when implied consent provisions have been met. In 2002, state BAC testing rates of surviving drivers ranged from less than 1% in Virginia to 90% in Maine (Hedlund et al., 2004
). For surviving drivers, BAC testing is largely the responsibility of law enforcement agents. States with the highest BAC testing rates are those with a mandatory testing law (Hedlund et al., 2004
). High BAC testing rates have also been achieved by states that rely on dedicated teams to investigate fatal crashes or require law enforcement agents to request a voluntary test from all surviving drivers not suspected of driving while impaired (DWI), as well as requesting a test of the remaining drivers under standard DWI procedures (Hedlund et al., 2004
To achieve high BAC test rates for fatal crashes, medical examiners, coroners and law enforcement agents must be highly trained, properly equipped, and understand the importance of BAC testing. If more states followed a standard practice of BAC testing of all drivers involved in a fatal crash, not only would it increase testing rates, but it would reduce the testing biases associated with current practices. A recent study by Hedlund et al. (2004)
that reviewed state laws and practices for BAC testing of drivers involved in fatal crashes recommended that NHTSA establish national guidelines for BAC testing and reporting rates. Based on the results states have been able to achieve nationwide, they recommended that the target BAC testing rate be 80% for fatally injured drivers and 60% for surviving drivers.
The results of this study must be interpreted in the context of the following limitations. First and most importantly, we were only able to evaluate the accuracy of police-reported judgments of alcohol involvement in a nonrandom sample, that is, only among the 25% of drivers for whom we also had a BAC test result. We suspect that police-reported judgments are less accurate in the absence of chemical testing because in more than one-third (39%) of the cases where both alcohol sources were present, the police reported basing their judgment on an evidential test.
Second, this study is limited to drivers involved in fatal motor carrier crashes. However, our results are consistent with studies involving other types of vehicles and drivers so the results are probably generalizable to crashes not involving a motor carrier.
A third limitation is that there may be other factors that influence the accuracy of police-reported judgments that we were unable to evaluate, such as the severity of the injury among surviving drivers. Past literature is inconsistent regarding whether injury severity significantly influences the accuracy of police-reported judgments of alcohol involvement (Grossman et al., 1996
; van Wijngaarden et al., 1995
). Finally, because of data limitations, our definition of a motor carrier was based solely on the gross vehicle weight rating. There are other classes of motor carriers as defined by the Federal Motor Carrier Safety Administration (2009)
that we were not able to classify as motor carriers with the FARS data (i.e., vehicles with a combination weight rating of greater than 26,000 pounds as well as vehicles carrying hazardous materials or more than 15 passengers).