How often do people drive impaired by multiple substances?
One in three Americans now lives in an area where cannabis consumption is legal. Additionally, greater use of prescription and over-the-counter medications, an ongoing opioid epidemic, and continued challenges with eliminating alcohol-impaired create a new and very dangerous road risk: Drivers impaired by multiple substances.
Driving impaired by one substance is dangerous but when substances are combined, there is a multiplicative effect on driver impairment. People who drive under the influence of alcohol and drugs are up to 200 times more likely to be involved in a crash (Shulze et al., 2012; Griffiths, 2014).
According to FARS data, in 2016, 50.5% of fatally-injured drug-positive drivers (with known drug test results) were positive for two or more drugs and 40.7% were found to have alcohol in their system (FARS as cited in Hedlund, 2018).
Recent data from Washington State revealed that multi-substance impairment was the most common type of impairment found among drivers involved in fatal crashes between 2008 and 2016 (Grondel et al., 2018). In fact, among drivers involved in fatal crashes during this timeframe, 44% tested positive for two or more substances with alcohol and THC being the most common combination (Grondel et al., 2018).
In an analysis of 2017 DUI case data, the Colorado Department of Public Safety reported a significant number of multi-substance cases but cautioned that the figures are likely an underrepresentation of the magnitude of the problem due to limited drug testing in cases where alcohol is present. In 2017, 13.6% (2,362) of cases with toxicology results had more than one drug present. In multi-substance cases, 40.6% (958) involved a combination of alcohol and cannabis and 18.9% (447) involved cannabis and an additional drug. A further 10.6% (251) of multi-substance cases involved a combination of alcohol, cannabis, and at least one other drug (Bui & Reed, 2019).
According to data reported by the Rocky Mountain High Intensity Drug Trafficking Area (RMHIDTA), in 2018 drivers who were involved in fatal crashes who tested positive for cannabis were far more likely to test positive for multiple substances as opposed to cannabis alone. In fact, 35% of these drivers tested positive for cannabis and alcohol, 29% tested positive for cannabis and other drugs, and 11% tested positive for a combination of cannabis, alcohol, and other drugs (RMHIDTA, 2019). This means that 75% of drivers involved in fatal crashes who tested positive for cannabis had multiple substances in their systems at the time of the crash.
The Driving under the Influence of Drugs, Alcohol and Medicines (DRUID) project of the European Commission identified the relative crash risk associated with various categories of drugs. The researchers found that the use of multiple drugs produces a ‘highly increased crash risk’ and the combination of alcohol and drugs produces an ‘extremely increased crash risk.’ According to this research, individuals who drive under the influence of alcohol and drugs are up to 200 times more likely to be involved in a crash (Shulze et al., 2012; Griffiths, 2014).
2018 data from the Drug Recognition Expert (DRE) Section of the International Association of Chiefs of Police (IACP) shows approximately 42% of all DRE enforcement evaluations (13,230 cases) included an opinion of multi-substance impairment. Of all the evaluations with available toxicology results, 36% were positive for multiple drugs (Hayes/IACP, 2019).