Many drivers who tested positive for drugs -- even when they are legal - didn't report taking them. Though not surprising, the finding underlines the importance of conducting blood or saliva tests.
That is the main take-away of a new study released earlier this month by the Insurance Institute for Highway Safety, a nonprofit financed by the insurance industry, that compared self-reported drug use within 24 hours of getting behind the wheel with biological drug test results.
The Insurance Institute's analysis was based on two national surveys of more than 7,000 drivers from 300 locations across the continental U.S.
“Although we expected drug use to be under reported, it was surprising how inconsistent the results were from one survey year to another,” Angela Eichelberger, senior research scientist for the institute and lead author of the paper, said in a statement. “These findings suggest that self-reported drug use is not a good measure for monitoring trends in drug use in this population.”
For the surveys, researchers worked with local police to stop drivers who were asked to participate anonymously in both biological tests and self-reports. (Drivers found to be impaired did not face charges and were provided with alternative transportation.)
Researchers looked specifically at the results related to cannabis, opioids, cocaine, antidepressants and benzodiazepines, which include drugs like Valium and Xanax.
The study said that researchers don’t have good data on the prevalence of drug-involved driving nationwide, as biological measures are not collected regularly, and the federal data on drug use is problematic. Many drivers involved in fatal crashes are never tested, and when they are, inconsistencies in testing procedures in different states and time periods make it difficult to accurately estimate trends.
The report also noted that 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, as sometimes chemical indicators of marijuana and other drugs can remain in the blood longer than that. In addition, not all of the drivers who reported taking drugs in the 24 hours prior to their participation in the survey tested positive.
The institute indicated that the discrepancies could reflect that drivers might have been mistaken about when they took the drug or about the kind they had taken, and different drugs take varying lengths of time to disappear from one’s system.
Previous studies by the Insurance Institute and its affiliate the Highway Loss Data Institute showed crashes increased as much as 6 % following the start of retail sales of recreational marijuana in Colorado, Nevada, Oregon and Washington.
The safety group said that both biological tests and self-reporting are critical to understand the extent of the drug-impaired driving problem. Blood and saliva tests provide important objective information, but driver feedback can provide details about drug use that the tests can’t measure, like frequency of use and mode of administration.
For more details about the report, click here.
