Studies suggest that poly-drug use or combining alcohol and drugs can inflate the level of driver impairment and crash risk. There is variation across jurisdictions in the frequency of testing suspected impaired drivers for drugs, consistency in laboratory drug testing practices, and capacity of law enforcement personnel. Despite challenges in identifying causality and impairment, there is general consensus that many drugs impair driving. Preliminary data from an ongoing NHTSA study of alcohol and drug prevalence during the COVID-19 emergency found some significant increases in the prevalence of drugs detected in blood among fatally and seriously injured drivers, motorcyclists, and pedestrians when comparing the last quarter of 2019 and the first quarter of 2020 to the second, third, or fourth quarters of 2020. As of February 2022, nearly three-quarters of states, including California, have legalized medical-use of cannabis products and over one-third allow recreational cannabis, increasing concerns about traffic safety. According to the National Institute on Drug Abuse, aside from alcohol, cannabis is the most frequently detected drug in drivers who are in crashes.
Analyses from FARS presented in the drug-impaired program area include fatalities in crashes that involved a driver who tested positive for a drug that could cause impairment. Analyses from SWITRS presented in this program area refer to drug-involvement and include fatal and serious injuries where law enforcement reported the driver to be under the influence of drugs. Crashes in the program area are defined as where one or more drivers tested positive for a drug that could cause impairment or was reported as driving under the influence of drugs, depending on which data set is used.