A few weeks ago, I noted that the Obama administration, reputedly more enlightened than its predecessor on matters of drug policy, is encouraging states to enact gratuitously punitive laws that treat drivers with marijuana metabolites in their urine as if they were drunk. Since traces of marijuana can be detected in urine long after the drug's effects have worn off, this policy is just an excuse for sending pot smokers to jail. I suggested that tests of THC in blood would be a more accurate measure of impairment, comparable to the standard for alcohol. But as at least one commenter noted, there is an argument for treating drivers under the influence of marijuana less severely than drivers under the influence of alcohol: They are less of a threat to public safety. Experiments repeatedly have found that marijuana has a less dramatic impact on driving ability than alcohol does, with the added advantage that pot smokers seem to be more aware of their impairment and therefore tend to compensate for it by slowing down (whereas drinkers tend to speed up). In the latest study, reported recently in the Journal of Psychoactive Drugs, subjects who smoked a joint containing marijuana with about 3 percent THC "decreased their speed [more than the sober control subjects did] and failed to show expected practice effects during a distracted drive," but "no differences were found during the baseline driving segment or collision avoidance scenarios."

In their book Marijuana Is Safer, which I reviewed in the April issue of Reason, Steve Fox, Mason Tvert, and Paul Armentano cite marijuana's relatively minor impact on driving ability as a public safety advantage. To the extent that legalizing pot encourages people to shift from alcohol to marijuana, it could actually produce a net decrease in traffic deaths, contrary to the nightmare scenarios painted by prohibitionists.

Armentano surveys the evidence on marijuana and driving here.

[via the Drug War Chronicle]