Driving, Drinking, and Drug Use

6. Estimated Risk of Motor Vehicle Accident Associated With Marijuana Use

"This study of crash risk found a statistically significant increase in unadjusted crash risk for drivers who tested positive for use of illegal drugs (1.21 times), and THC specifically (1.25 times). However, analyses incorporating adjustments for age, gender, ethnicity, and alcohol concentration level did not show a significant increase in levels of crash risk associated with the presence of drugs. This finding indicates that these other variables (age, gender ethnicity and alcohol use) were highly correlated with drug use and account for much of the increased risk associated with the use of illegal drugs and with THC.
"This study found a statistically significant association between driver alcohol level and crash risk both before and after adjustment for demographic factors. These findings were generally consistent with similar analyses conducted in prior crash risk studies. Findings from this study indicate that crash risk grows exponentially with increasing BrAC. The study shows that at low levels of alcohol (e.g., 0.03 BrAC) the risk of crashing is increased by 20 percent, at moderate alcohol levels (0.05 BrAC) risk increases to double that of sober drivers, and at a higher level (0.10 BrAC) the risk increases to five and a half times. At a BrAC of 0.15, the risk is 12 times, and by BrACs of 0.20+ the risk is over 23 times higher."

Compton, R. P. & Berning, A. (2015, February). Drug and alcohol crash risk. (Traffic Safety Facts Research Note, Report No. DOT HS 812 117). Washington, DC: National Highway Traffic Safety Administration, p. 8.

7. Estimated Risk Of Motor Vehicle Accident Associated With Marijuana Use

"Conducting case-control studies to estimate the risk of crash involvement from drug use presents many difficulties. The first challenge has been getting reliable and accurate estimates of drug use. Many studies rely on self-report (which have obvious inherent problems) rather than actual measurement of THC in blood or oral fluid.
"Also, the method of selecting truly comparable control drivers in an unbiased fashion for the crash involved drivers has varied considerably. The more carefully controlled studies, that actually measured marijuana (THC) use by drivers rather than relying on self-report, and that had more actual control of covariates that could bias the results, generally show reduced risk estimates or no risk associated with marijuana use (Elvik, 2013)."

Compton, R. P. & Berning, A. (2015, February). Drug and alcohol crash risk. (Traffic Safety Facts Research Note, Report No. DOT HS 812 117). Washington, DC: National Highway Traffic Safety Administration, pp. 1-2.

8. Odds Of Involvement In Fatal Auto Accidents Associated With Use Of Various Substances

"The prevalence of drugs detected in cases was higher than in controls across the drug categories (Table 3). Marijuana, narcotics, stimulants, and depressants were each associated with a significantly increased risk of fatal crash involvement, with estimated odds ratios ranging from 1.83 for marijuana to 4.83 for depressants (Table 3). Polydrug use, defined as use of two or more non-alcohol drugs, was associated with a 3.4-fold increased risk of fatal crash involvement (Table 3).
"About one-fifth (20.5%) of the cases tested positive for alcohol and one or more drugs, compared with 2.2% of the controls. Relative to drivers who tested positive for neither alcohol nor drugs, the estimated odds of fatal crash involvement increased over 13 folds for those who were alcohol-positive but drug-negative, more than two folds for those who were alcohol-negative but drug-positive, and 23 folds for those who were positive for both alcohol and drugs (Table 4)."

Guohua Li, Joanne E. Brady, and Qixuan Chen. Drug use and fatal motor vehicle crashes: A case-control study. Accident Analysis and Prevention 60 (2013) 205–210. http://dx.doi.org/10.1016/j.aa....

9. Cannabis Consumption, THC Levels, Performance Impairment

"It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. Concentrations of parent drug and metabolite are very dependent on pattern of use as well as dose. THC concentrations typically peak during the act of smoking, while peak 11-OH THC concentrations occur approximately 9-23 minutes after the start of smoking. Concentrations of both analytes decline rapidly and are often < 5 ng/mL at 3 hours. Significant THC concentrations (7 to 18 ng/mL) are noted following even a single puff or hit of a marijuana cigarette. Peak plasma THC concentrations ranged from 46-188 ng/mL in 6 subjects after they smoked 8.8 mg THC over 10 minutes. Chronic users can have mean plasma levels of THC-COOH of 45 ng/mL, 12 hours after use; corresponding THC levels are, however, less than 1 ng/mL. Following oral administration, THC concentrations peak at 1-3 hours and are lower than after smoking. Dronabinol and THC-COOH are present in equal concentrations in plasma and concentrations peak at approximately 2-4 hours after dosing.
"It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH concentrations. It is possible for a person to be affected by marijuana use with concentrations of THC in their blood below the limit of detection of the method. Mathematical models have been developed to estimate the time of marijuana exposure within a 95% confidence interval. Knowing the elapsed time from marijuana exposure can then be used to predict impairment in concurrent cognitive and psychomotor effects based on data in the published literature."

Couper, Fiona J., Logan, Barry K., et al., "Drugs and Human Performance Fact Sheets," (Washington, DC: National Highway Traffic Safety Administration, April 2004), pp. 8-9.

10. Medical Marijuana Laws Associated With Reduction in Traffic Fatalities

"Using population-based data from 1985 to 2014, we found that, first, states that enacted MMLs during the study period had lower fatality rates compared with states without MMLs. Second, on average, traffic fatalities further decreased in states post-MML, with both immediate (sudden change in fatality rate after MML enactment) and gradual (change in rate trend after MML enactment) declines over time in those aged 25 to 44 years. Third, the association between MML and traffic fatalities varied considerably across states. Fourth, the presence of operational dispensaries was also associated with reductions in traffic fatalities in those aged 25 to 44 years.
"We found that, on average during the study period, MML states had lower traffic fatality rates than non-MML states. It is possible that this is related to lower levels of alcohol-impaired driving behavior in MML states. Evidence from the Behavioral Risk Factor Surveillance Systems data from 200027 and 201228 shows that states that have enacted MMLs, compared with non-MML states, had, on average, lower proportions or rates of drivers endorsing having driven after having too much to drink. In addition, other unmeasured characteristics, including strength of public health laws related to driving, infrastructure characteristics (e.g., high-technology roads), or quality of health care systems, may partially explain these findings."

Julian Santaella-Tenorio, Christine M. Mauro, Melanie M. Wall, June H. Kim, Magdalena Cerdá, Katherine M. Keyes, Deborah S. Hasin, Sandro Galea, and Silvia S. Martins. US Traffic Fatalities, 1985–2014, and Their Relationship to Medical Marijuana Laws. American Journal of Public Health: February 2017, Vol. 107, No. 2, pp. 336-342.
doi: 10.2105/AJPH.2016.303577