Is it Safe to Be High on the Highway?

In case you haven’t heard, California’s Proposition 19 was defeated last week. The initiative, which would have revoked state laws that prohibit the possession and small-scale cultivation of marijuana, drew only 46 percent of the vote. I’m no expert on what this will mean for incarceration rates, California’s budget, drug-related violence or the sales of Phish concert tickets, but what will keeping pot illegal mean for transportation?

A study by the RAND Corporation concluded that passage of Prop 19 would have reduced pretax marijuana prices by something on the order of 80 percent. Chapter one, page one of your microeconomics textbook tells you that consumption of a good rises as its price drops. So it’s probably safe to say that cannabis use would have gone up, perhaps considerably (though RAND could not hazard a guess about the magnitude of the increase). This would certainly have translated into more driving under the influence of pot. And obviously, this would have led to more dangerous driving, more accidents and more carnage on our roads. Maybe.

As a recent review of the evidence by R. Andrew Sewell, James Poling and Mehmet Sofuoglu (all of Yale University’s School of Medicine) demonstrates, the links between cannabis use and dangerous driving are more complex than one might think.

One thing is certain: marijuana does erode your driving skills. Past work shows that “attentiveness, vigilance, perception of time and speed, and use of acquired knowledge” are all impaired by pot. In fact, an analysis of 60 studies (performed by G. Berghaus and B. Guo) found that pot impairs you “in every performance area that can reasonably be connected with safe driving of a vehicle, such as tracking, motor coordination, visual functions, and particularly complex tasks that require divided attention.”

Yet despite the fact that driving while high is self-evidently a terrible idea, the evidence on whether stoned drivers are indeed a menace to society is surprisingly mixed. How can this be?

Surprisingly, most lab experiments show that stoned drivers perform little worse than sober ones, except when pot is combined with alcohol. Many researchers have concluded that this is because drivers who are high are very aware that they are impaired, and deal with their neurological and psychological deficiencies by adopting coping mechanisms to compensate – even overcompensate – for their altered state. This leads to stoned drivers driving at lower speeds, leaving greater following distances between cars and making fewer efforts to pass other vehicles.

Interestingly, it seems that pot impairs “automatic” functions, like reaction time or the ability to stay in one’s lane, while leaving “cognitive” functions (e.g. the choice of speed) comparatively intact. Alcohol, on the other hand, degrades cognitive function, with drunks much more prone to making risky choices behind the wheel.

Of course, lab experiments may be a poor reflection of real-life driving behavior: after all, subjects know they’re being studied. Lab work is a better reflection of what people can do than what they actually do.

So numerous studies have looked at pot’s effects on actual driving behavior. This is primarily done using two methods. One is culpability study, which determines which driver was at fault in an accident and whether the guilty party was more likely to be stoned than the innocent one.? Some of these inquiries have found drivers who are high are more likely to be at fault in crashes, but many conclude that stoned drivers are no more dangerous – and indeed might be less dangerous – than others.

The ambiguity in the literature is due to numerous factors. For example, the mechanics of testing for pot use are tricky. Also, even when elevated accident levels for pot users have been documented, it could be argued that this is because they tend to share certain common characteristics – youth, male gender, a predisposition to risk-taking – that are more responsible for the accidents than the pot itself. A couple of studies suggest that this is the case. In all, the jury’s still out.

The other main method for studying this issue is the case control technique. As the name suggests, stoned drivers are compared with an appropriate control group. The trick is finding that control group, and being sure it matches well with the stoned driver group, something most studies to date have often not done satisfactorily. In all, as with culpability studies, the results from case control studies have been inconclusive.

There are three things, however, that we do know conclusively. First, driving stoned is not “safe;” impairment unquestionably exists. Sewell et al. suggest that “patients who smoke marijuana should be counseled to have a designated driver if possible, to wait at least three hours after smoking before driving if not, that marijuana is particularly likely to impair monotonous or prolonged driving, and that mixing marijuana with alcohol will produce much more impairment than either drug used alone.”

Second, the kind of ambiguous results we get for stoned drivers have never been found with respect to drunk drivers, who have proven conclusively and ad nauseum to be a serious menace, as I’ve written about extensively.

Finally, since the adaptive techniques used by stoned drivers – sticking to the speed limit, leaving plenty of room between you and the car in front of you, foregoing gratuitous passing of other vehicles – seem to be effective, think of what a better place the world would be if we all adopted them – while driving sober.