When asked about drug use, drivers' answers often contradict tests

September 17, 2019

(Pacific Institute for Research and Evaluation)

Pacific Institute for Research and Evaluation

A new study comparing self-reported drug use with drug test results has confirmed what most people might expect: Drivers aren’t especially eager to admit taking drugs within 24 hours of getting behind the wheel — even when those drugs are legal.

Based on an analysis of the two most recent national roadside surveys, a new study from IIHS found that far from all drivers who later tested positive for drugs had reported taking them in their responses to questionnaires. The finding underlines the importance of conducting blood or saliva tests to understand the extent of the drug-impaired driving problem.

“Although we expected drug use to be underreported, it was surprising how inconsistent the results were from one survey year to another. These findings suggest that self-reported drug use is not a good measure for monitoring trends in drug use in this population,” says IIHS Senior Research Scientist Angela Eichelberger, the lead author of the paper.

Currently, researchers simply don’t have good data on the prevalence of drug-involved driving nationwide, she notes. Biological measures are not collected regularly, and the information on drug use from the Fatality Analysis Reporting System, a census of fatal crashes on U.S. roads, is problematic. Many drivers involved in fatal crashes are never tested for drugs, and when they are tested, inconsistencies in testing procedures among different states and different time periods make it difficult to accurately estimate trends.

A potentially better source of data is the national roadside survey of alcohol and drug use by drivers, which has been conducted five times since it began in 1973. To gather results, researchers work with local police to safely stop drivers, who are asked to participate anonymously. Drivers who are found to be impaired do not face charges. However, they are provided with alternative transportation.

The first three times it was conducted, survey workers used breath testing and a brief interview to estimate the prevalence of alcohol-impaired driving. In the most recent two surveys, workers asked about drug use too. They also collected saliva and blood samples, and those who agreed to provide them received a small payment.

Researcher giving driver a breath test
Driver filling out survey form
A driver participates in a roadside survey of alcohol and drug use. (Pacific Institute for Research and Evaluation)

In the 2007 and 2013-14 surveys, workers collected anonymous data from 300 locations across the continental U.S., gathering both biological and self-reported information from more than 7,000 drivers during each survey. For the IIHS study, researchers looked specifically at the results related to cannabis, opioids, cocaine, antidepressants and benzodiazepines, which include such drugs as Valium and Xanax.

Results varied for the different types of drugs. In both surveys, fewer than 1 in 5 respondents who tested positive for cocaine reported taking it within the past 24 hours in their answers to the questionnaire. The proportion of marijuana-positive drivers who said they had taken the drug within the past 24 hours increased from a quarter to nearly 40 percent between 2007 and 2013-14 — likely due to broader social acceptance following its legalization in several states.

For prescription drugs, the reporting rate was somewhat higher: more than 40 percent for opioids and more than 70 percent for antidepressants in both surveys and better than half for benzodiazepines in 2013-14. These higher rates may reflect lower social stigma associated with these drugs or less fear of legal consequences.

It’s possible that some drivers who tested positive but said they hadn’t used drugs in the past 24 hours were telling the truth. Chemical indicators of marijuana and other drugs can in some circumstances remain in the blood longer than that.

Curiously, not all of the drivers who reported taking drugs over the past 24 hours tested positive. Only 21 percent of respondents who reported taking antidepressants in 2013-14 tested positive, for instance, most likely because the biological tests didn’t cover the complete range of those medications. On the other hand, 64 percent of respondents who admitted using cocaine and 82 percent who admitted using marijuana tested positive for those drugs.

“The discrepancies could indicate that drivers might have been mistaken about when they took the drug or mistaken about the kind of medication they had taken,” Eichelberger says. “These drugs also take varying lengths of time to disappear from your system.”

That means both biological testing and self-report data can be useful. The biological tests provide important objective information. But the self-reports can provide details about drug use that the tests can’t measure, such as frequency of use and mode of administration.

Researchers are eager for more robust data than are currently available. Almost a third of all motor vehicle fatalities have been alcohol-related for more than a decade, but the most recent national roadside surveys showed a drop in the proportion who tested positive for alcohol and an increase in the proportion who tested positive for illegal drugs and medications.

Although roadside surveys are useful for tracking changes in drug use among drivers, it is not clear how many of the drivers were impaired by drugs other than alcohol, since there are no evidence-based standards to establish impairment based on a drug test.

Meanwhile, trends like the increasing acceptance of marijuana and the opioid crisis make it even more important to develop a better understanding of drug-impaired driving.

Studies by IIHS and HLDI, for instance, showed crashes increased as much as 6 percent following the start of retail sales of recreational marijuana in Colorado, Nevada, Oregon and Washington (see “Crashes rise in first states to begin legalized retail sales of recreational marijuana,” Oct. 18, 2018).

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