How bad is stoned driving, and what should we do about it?

“Stoned driving” isn’t nearly as bad as drunk driving. How about a zero BAC limit when THC is present?

Dishonest advocacy aside, what are the actual risks of stoned driving? The answer, from what seems to be a well-done case-control study, is that driving stoned is hazardous, but much less hazardous than driving drunk. (A relative risk of 1.83 – meaning that driving a mile stoned is about as risky as driving two miles sober – strongly suggests that cannabis-impaired driving is a problem, but also that it isn’t much of a problem; the relative-risk number for alcohol is over 13.) On the other hand, the same study shows that adding cannabis or other drugs to alcohol substantially worsens the odds: alcohol-and-something-else has a relative risk of 23.

Given those numbers, and the technical difficulty of identifying cannabis-impaired driving (because impairment doesn’t track cannabinoid levels in blood nearly as well as it tracks alcohol levels) I’d propose the following rule: anyone who tests positive for cannabis on a mouth swab (which detects use within the past few hours) should be considered guilty of impaired driving if that person’s BAC is detectably different from zero. All that means is that, if you’ve been toking and drinking, you need to wait as many hours as you’ve had drinks before getting behind the wheel.

Of course, if I were advising someone personally, I’d be much more cautious: driving within six hours of using cannabis is pretty damned stupid. But the same is true of driving after a sleepless night. The question isn’t what’s imprudent; the question is what’s hazardous enough to make a serious criminal offense. So far, the numbers I see about stoned driving (in the absence of alcohol) don’t bring it across that very high threshold.

Given the long latency of THC, the “zero-tolerance” rules now being passed in some states, which making driving with any detectable cannabis on board drugged driving per se, without evidence of actual impairment, are simply a backdoor way of recriminalizing cannabis use.

Footnote A case-control study – comparing a group of drivers responsible for accidents with a random sample of all drivers – is the only way to figure out what’s really going on. The observation that, as cannabis use spreads, more crash-responsible drivers have cannabis on board tells you precisely nothing. After all, if consumption of blintzes were to increase, more responsible drivers would test positive for ricotta. You neeed a denominator, not just a numerator.

Author: Mark Kleiman

Professor of Public Policy at the NYU Marron Institute for Urban Management and editor of the Journal of Drug Policy Analysis. Teaches about the methods of policy analysis about drug abuse control and crime control policy, working out the implications of two principles: that swift and certain sanctions don't have to be severe to be effective, and that well-designed threats usually don't have to be carried out. Books: Drugs and Drug Policy: What Everyone Needs to Know (with Jonathan Caulkins and Angela Hawken) When Brute Force Fails: How to Have Less Crime and Less Punishment (Princeton, 2009; named one of the "books of the year" by The Economist Against Excess: Drug Policy for Results (Basic, 1993) Marijuana: Costs of Abuse, Costs of Control (Greenwood, 1989) UCLA Homepage Curriculum Vitae Contact: Markarkleiman-at-gmail.com

2 thoughts on “How bad is stoned driving, and what should we do about it?”

  1. "On the other hand, the same study shows that adding cannabis or other drugs to alcohol substantially worsens the odds: alcohol-and-something-else has a relative risk of 23."

    Is this because of some special combination effect of alcohol and cannabis, or because a person that has taken both drugs has on average taken more "units of intoxication", under some reasonable definition of unit? Meaning, would a person that has consumed one unit of alcohol be more likely to have an accident if they afterwards took a unit of cannabis versus a second unit of alcohol?

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