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NHTSA |
Background and Introduction 
While the extent of use of alcohol by drivers and the risks posed by alcohol use have been well known for many decades, relatively little has been known about the use of other drugs by drivers and the associated risks. However, drug-impaired driving has recently become an issue of increasing public and governmental concern in the United States and in many other countries (Compton et al., 2009; Asbridge et al., 2012; ICADTS, 2007). While it is readily apparent that driving-related skills can be impaired by a wide variety of illegal substances and medications, the nature and scope of the drug-impaired driving problem has been difficult to define (Jones et al., 2003; DuPont et al., 2012; Houwing, 2013). In the United States, recent State actions to legalize the use of marijuana for medical and recreational use have further exacerbated concern over potential risks of driving impaired by marijuana.

Marijuana is the most frequently detected drug (other than alcohol) in crash-involved drivers as well as the general driving population (Terhune, 1982; Terhune et al., 1992; Lacey et al., 2009; Walsh et al., 2005). There is evidence that marijuana use impairs psychomotor skills, divided attention, lane tracking, and cognitive functions (Robbe et al., 1993; Moskowitz, 1995; Hartman and Huestis, 2013). However, its role in contributing to the occurrence of crashes remains less clear. Many studies, using a variety of methods have attempted to estimate the risk of driving after use of marijuana (Li et al., 2012; Asbridge et al., 2012). The methods have included experimental studies, observational studies, and epidemiological studies. While useful in identifying how marijuana affects the performance of driving tasks, experimental and observational studies do not lend themselves to predicting real world crash risk.

Epidemiological Studies
Epidemiological studies differ in how they estimate risk. Culpability studies compare the rate at which crash-involved, drug-positive drivers and drug-negative drivers are deemed to be at fault for their crashes. Case-control studies compare drug use by crash-involved drivers to drug use by non-crash involved drivers. In general, the case-control method is preferable since it can eliminate more sources of potential bias in estimating crash risk resulting from drug use (e.g., alcohol use is much higher at night and on weekends than during the day or on weekdays). The existing epidemiological research (both culpability and case-control studies) have produced contradictory estimates of risk for marijuana use. Some of these studies have suggested that marijuana use has minimal or no effect on the likelihood of crash involvement, while others have estimated a small increase in the risk of crash involvement.

Two recent population-based case control studies have estimated the crash risk of drug use by drivers by using NHTSA’s Fatality Analysis Reporting System (FARS) 2007 data for the crash-involved driver population and the 2007 National Roadside Survey of Alcohol and Drug Use by Drivers for the control drivers (Lia, Bradya, & Chen, 2013; Romano, Torres-Saavedra, Voas, & Lacey, 2014). The Li study estimated the increased risk of crash involvement for drivers using marijuana at 1.83 times that of drug-free drivers, while the Romano study found no increased risk of crash involvement for those drivers testing positive for THC (the main psychoactive substance in marijuana). However, current limitations in the FARS dataset do not allow calculation of unbiased, reliable and valid estimates of the risk of crash involvement that results from drug use.