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The impairing effects of alcohol and the dangers of drinking and driving are well-documented. By contrast, there is considerably less research investigating the potentially impairing effects of drugs 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 impair driving. Moreover, the list of drugs in common usage is constantly changing.
  • 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 leaves the body in a predictable pattern, whereas other drugs are eliminated at many rates; hence, timing is critical when conducting drug tests. In addition, blood levels of certain drugs can accumulate with repeated administrations, and can 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 drugs with alcohol. Although individual drugs, taken at normal doses, may not impair driving, drug effects may be synergistic when taken together and substantially increase the risk of a crash.
  • 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 saliva.
  • Improvements to the quality and type of data collected during drug-impaired driving incidents are still in the initial stages of development and adoption by States and agencies.
  • Countermeasures for addressing potential driving impairments from prescription and over-the-counter drugs may need to be different than countermeasures for alcohol- and illicit drug-impaired driving.

Despite these challenges, a growing body of research suggests that many illicit, prescription, and over-the-counter drugs may impair a driver’s ability to operate a vehicle (for reviews, see Couper & Logan, 2004; Jones et al., 2003; Kelly et al., 2004; and Strand et al., 2016). Much of this research has involved laboratory or experimental studies using driving simulators, although some epidemiological studies have examined the effect of drugs on crash prevalence and risk. See Compton et al. (2009) for a discussion of this research.

In most cases, the research investigating the effect of drugs on driving has had variable results, in large part depending on the methodology employed. The crash risk associated with specific types of drugs is summarized below.

  • Benzodiazepines: Common benzodiazepines include Valium, Xanax, and Klonopin. Several studies suggest benzodiazepine users are at increased risk of being involved in a crash (Movig et al., 2004; Rapoport et al., 2009), although some studies have not found these results. The risk appears to depend on the type of benzodiazepine used, the dose, the time since last use, and whether the drug was combined with alcohol (Dassanayake et al., 2011; Leung, 2011).
  • Marijuana: The findings for marijuana also have been mixed, although a meta-analysis of epidemiological data concluded marijuana doubles the risk of a property damage or fatal crash (Asbridge et al., 2012). However, another study found only a 50% increase in the risk of property damage crashes, and no increase in the risk of fatal or injury crashes (Elvik, 2013). A large-scale study in Virginia found no elevated crash risk for THC users after adjusting for demographic variables and alcohol use (Compton & Berning, 2015). Generally, the risk appears highest when marijuana has been used recently, and especially when marijuana is combined with alcohol (Beirness & Simpson, 2006; Sewell et al., 2009). Pre-/post-studies on traffic safety related to marijuana legalization were conducted in Colorado and Washington. Both States legalized recreational marijuana use by adults 21 and older in 2014. A report from the National Bureau of Economic Research found that there was no firm evidence that the presence of THC was related to changes in marijuana-involved fatal crash risk in a comparison study between the two States and other control States between 2000 and 2016 (Hansen et al., 2018). However, some trends pointed to the potential for detrimental effects on safety. Fatal crashes involving marijuana increased in both States and THC-positive drivers involved in crashes increased in Washington (20% to 30% between 2005-2014); however, impairment status is unknown for these drivers.
  • Stimulants: There have been fewer studies examining the risks of stimulants such as amphetamines and cocaine on driving. The available studies suggest stimulants are strongly associated with fatal crashes (Elvik, 2013).
  • Narcotics: Several studies have showed that narcotic drugs such as morphine, heroin, and opiates increase crash risk. One case-control study found a three times higher risk of a fatal crash when a driver is under the influence of a narcotic (Li et al., 2013). However, this study used FARS data that have  limitations with respect to the interpretation, reporting, and testing of drug impairment in fatal crashes (Berning & Smither, 2014).
  • Antihistamines: The relationship between antihistamines and motor vehicle crashes is ambiguous (Moskowitz & Wilkinson, 2004). A small connection has been found between first-generation antihistamines and crashes, but second-generation antihistamines appear to cause less sedation.
  • Antidepressants: Second-generation antidepressant medications such as selective serotonin reuptake inhibitors do not seem to impair driving performance, but this is not necessarily the case with older types of antidepressants (Brunnauer & Laux, 2013).

Compton et al. (2009) describe four basic issues that must be addressed to better understand the extent of the problem of drug-impaired driving:

  • What drugs impair driving ability?
  • What drug dose levels are associated with impaired driving?
  • How frequently are impairing drugs being used by drivers?
  • What drugs are associated with higher crash rates?

In sum, there are still sizeable gaps in our understanding of the effects of drugs on driving. In their review of drug-impaired driving, Jones et al. (2003) concluded: “The role of drugs as a causal factor in traffic crashes involving drug-positive drivers is still not understood… Current research does not enable one to predict with confidence whether a driver testing positive for a drug, even at some measured level of concentration, was actually impaired by that drug at the time of crash” (p. 96). Perhaps the one consistent finding across studies is the risk of driver impairment increases substantially when drugs are combined with alcohol.

Similar to alcohol-impaired driving, drug-impaired driving is primarily addressed through a combination of laws, enforcement, and education (AAAFTS, 2018b; AAAFTS, 2018c). Relatively few countermeasures have been developed to specifically address drug-impaired driving, and there has been little evaluation of drug-impaired-driving countermeasures. The AAA Foundation for Traffic Safety investigated the potential for alcohol-impaired driving countermeasures to be applied to drugged and drug-impaired driving. Subject matter experts from across the United States participated in five panel discussions that were then summarized in two brief reports. The reports summarize the strengths and limitations of a range of countermeasures, including behavioral and educational interventions (AAAFTS, 2018b) and enforcement- and legal and policy-interventions (AAAFTS, 2018c). The conclusions point to the need for more research to better understand the nature and degree of traffic safety risk posed by drugs, as well as the effectiveness of potential countermeasures to address this issue. See the guide on drug-impaired driving produced by the Center for Problem-Oriented Policing for more information about drug-impaired-driving countermeasures (Kuhns, 2012). Marijuana-specific summaries can be found in NHTSA’s Report to Congress (Compton, 2017) and the AAA Foundation for Traffic Safety’s report (Logan et al., 2016). Smith et al. (2018) review the state of knowledge on countermeasures against impaired driving due to prescription and over-the-counter drugs.

Arnold & Scopatz (2016) provide 12 recommendations to States to address barriers to collecting and maintaining drug-impaired driving data. Based on these recommendations, an assessment of data collection procedures from across all States was performed (Fell et al., 2018). As of 2018 at least two-thirds of LEOs in 37 States had completed SFST training. Forty-nine States permit blood collection for drug testing, 35 States permit breath testing, and 39 States permit urine collection for drug testing. Fifteen States permit collection of oral fluids for drug testing and an additional 10 States had pilot test programs in place for oral fluid testing. Two States—Maryland and Washington—legally distinguish between arrests due to DUI-alcohol and DUI-drugs. An additional 32 States and the District of Columbia also report DUI-alcohol and DUI-drugs arrests separately.