In addition to alcohol, many drugs, both licit and illicit are known or suspected of impairing a driver’s ability to operate a motor vehicle safely (National Institute on Drug Abuse, 2019). The impairing effects of alcohol and the dangers of drinking and driving are well-documented. By contrast, there is considerably less research regarding the potentially impairing effects of drugs, other than alcohol, on drivers. Some of the challenges in studying, measuring, and creating countermeasures to address drug-impaired driving include the following (Arnold & Scopatz, 2016; Berning & Smither, 2014; Compton et al., 2009; Compton, 2017; Logan et al., 2016; Smith et al., 2018; Stewart, 2006):
- There is a wide range of drugs, both licit and illicit, that can potentially impair driving. Moreover, the most commonly used drugs constantly change.
- Although the relationship between BAC and driving impairment is clear and well-documented, the relationship between blood levels of drugs and driving impairment has not been established for drugs other than alcohol.
- Alcohol metabolizes in the body in a predictable pattern, whereas other drugs are eliminated at different rates and with variability across people; hence, timing is critical when conducting a drug test. In addition, blood levels of certain drugs can accumulate with repeated administrations and may be detected well after impairment has ceased.
- It is not unusual for drivers to take more than one impairing drug at the same time or to combine other drugs with alcohol. Even individual drugs that are usually non-impairing at therapeutic doses may substantially increase the risk of the crash when combined with alcohol or other drugs due to their synergistic effects. Furthermore, even at therapeutic doses, certain drugs have the potential to impair.
- Alcohol can be measured reliably through breath tests, but other types of drugs can only be measured through more intrusive tests of bodily fluids such as blood, urine, or oral fluid.
- Improvements to the quality and type of data collected during drug-impaired-driving incidents are in initial stages of development and adoption by States and government agencies.
- Evidence-based countermeasures addressing alcohol-impaired driving are likely effective against driving while under the influence of other drugs; however, the challenges associated with understanding the problem of drug-impaired driving and detecting drug-impaired drivers, such as the policy of some agencies to conduct no further drug testing if alcohol is at or above the illegal per se limit, warrant a separate discussion.