An article in the January 2014 issue of The Journal of Studies on Alcohol and Drugs by Eduardo Romano and colleagues titled "Drugs and Alcohol: Their Relative Crash Risk" compares the differential effects of drugs and alcohol on the probability of being involved in a fatal automobile accident. The U.S. Fatality Analysis Reporting System (FARS) and the National Roadside Survey (NRS) were the sources of the data analyzed in this study. The researchers conducted two studies of this data. One study compared the relative risk of alcohol with the relative risk of all other drugs. The second analysis separated marijuana out and made a three way comparison of the relative risks of marijuana, alcohol, and other drugs.
When alcohol was compared to all other drugs, including marijuana, the following results were obtained:
Both alcohol and drugs significantly increased the odds of being involved in a fatal traffic accident, as did a combination of drugs and alcohol. Around 58% of drivers involved in traffic fatalities were positive for alcohol whereas around 20% were drug positive. Drivers from the National Roadside Survey were used as a control group and it was found that 9% of control drivers were positive for alcohol and 13% were positive for drugs. These data allow Odds Ratios to be calculated. The unadjusted Odds Ratio of a drugged driver being involved in a traffic fatality is 1.66 whereas the unadjusted Odds Ratio of a drunk driver being involved in a traffic fatality is 14.67. These are both statistically significant; however, the Odds Ratio for the drunk driver is nine times greater than that of the drugged driver. Interestingly, there was no significant increase in risk for drivers who had mixed drugs and alcohol as compared to drivers who tested positive for alcohol alone.
When marijuana was separated out and a three way comparison was made between marijuana, alcohol, and other drugs, the following results were obtained:
Drivers who tested positive for marijuana were at no greater risk for an automobile fatality than were drivers who tested negative for all substances. However, the authors note that this does NOT lead to the conclusion that it is safe to drive in a state of marijuana intoxication. People continue to test positive for marijuana many days after ingesting it, long after the state of marijuana intoxication has passed. A 2004 study by JG Ramaekers suggests that acute cannabis intoxication can cause major impairment of one's ability to operate a motor vehicle, whereas the presence of THC from past cannabis intoxication does not. There is a good chance that the majority who tested positive for marijuana were not acutely intoxicated but rather were past users.
The authors also noted that there was a significant interaction between age and BAC in the drivers who tested positive for alcohol, younger drivers were at significantly greater risk for traffic fatalities than were older drivers. This is illustrated in the following table.
Finally, the authors of this study call for further investigation of more data to determine the differential effects of different drug classes: stimulants, depressants, narcotics, and other.
This study leads to the conclusion that we should focus strongly on drunk driving prevention, particularly among younger drivers. I propose that one of the most effective means of reducing both drunk driving and drugged driving is to reduce dependence on the automobile and driving in general. Raising gasoline taxes and using the revenue to build public transportation is one strategy. Closing more city streets to traffic other than busses and taxis is another. And how about making it illegal for a bar to have a parking lot? Having a parking lot for a bar is just asking for trouble.