Archive for October 7th, 2009

I eagerly started reading SuperFreakonomics last night, and wasn’t long into before I encountered a rather eye-raising bit of traffic-related material (which is of course not a surprise).

The authors, Steven Levitt and Steven Dubner, write:

“Each year, more than 1,00 drunk pedestrians die in traffic accidents. They step off sidewalks into city streets; they lie down to rest on country roads; they make mad dashes across busy highways. Compared with the total number of people killed in alcohol-related traffic accidents each year — about 13,000—the number of drunk pedestrians is relatively small. But when you’re choosing whether to walk or driver, the overall number isn’t what counts. Here’s the relevant question: on a per-mile basis, is it more dangerous to drive drunk or walk drunk?

After running through some numbers, they find:

“Doing the match, you find that on a per-mile basis, a drunk walker is eight times more likely to get killed than a drunk driver.

They add a caveat that drunk walkers don’t kill other people, as drunk drivers do; but even factoring for that, “walking drunk leads to five times as many deaths per mile as driving drunk.”

As the jacket notes, the book asks “not only the tough questions, but the unexpected ones.” But having raised this rather startling statistic, it then moves on, leaving a number of interesting questions in its wake. The first is the problem with exposure data (not just how much but when and where), which for pedestrians is notoriously inaccurate, and for drivers only slightly better. Another thing that seemed worth raising immediately is the idea of causality; not that Levitt and Dubner have done this, but one might walk away from those numbers with the idea that all those pedestrians were running heedlessly drunk into the streets. But the statistic simply refers to the blood-alcohol content of pedestrians killed in traffic, so one could be standing on a street corner after having had a few drinks, and be killed by an out-of-control taxi (or a hit-and-run driver), and this would be coded as a pedestrian alcohol-impaired crash (One could also be walking to one’s car, of course, as many “pedestrians” in America are doing, although the authors present an either/or case of walking or driving).

And of course, the relationship in the book between drunk drivers and drunk walkers is perhaps less dramatic when viewed in light of the overall risk of walking versus driving. As a paper by John Pucher and Lewis Dijkstra notes, “it is much more dangerous to walk or cycle in American cities than to travel by car. Per kilometer traveled, pedestrians were 23 times more likely to get killed than car occupants in 2001 (140 vs 6 fatalities per billion kilometers), while bicyclists were 12 times more likely than car occupants to get killed (72 vs 6 fatalities per billion kilometers).”

This raises the question of how much more of a risk it is to walk drunk than to walk sober — which perhaps is the more interesting point of comparison here. But that question is of course complicated by any number of factors, like exposure — pedestrian deaths that involve alcohol are overrepresented at night, when visibility is a greater problem (and there are also more drunk drivers on the roads — and as this chart notes, a certain number of pedestrian fatalities involve alcohol impairment on both the part of the driver and the pedestrian). Another factor to consider, one not addressed by the authors, is the number of fatalities by .BAC level. Particularly in the category in which no driver alcohol impairment was implicated (but in other categories as well), there are many more pedestrians killed at at the .10+ BAC level than at the .01-.09 level (I’m not sure how this mathematically matches up with driver deaths by BAC level). So how much one has had to drink (which is probably associated with all sorts of other factors, like time of day, etc.) matters as well.

There’s a lot of other interesting things to consider here. One would be the mindset of severely intoxicated pedestrians versus drivers. One imagines, at least among a certain part of the intoxicated driving public, a certain behavioral adaptation (even if their performance per se cannot be modified), to avoid getting caught. As walking intoxicated is generally not against the law, is there greater risk-taking behavior at work? Certainly there are demographic factors to consider as well — it’s hard to imagine that drunk drivers and drunk walkers sync up in terms of life profile in most places in America. Then there’s the question of facilities — how many alcohol-impaired pedestrians were killed walking in unsafe places, places without sidewalks, etc.? (presumably stumbling through the French Quarter is safer than trying to leave a rural or exurban bar on foot, etc.). And I’m sure there are things I’ve left out that spring to your mind.

In any case, if it’s a sad commentary that driving drunk would be safer than walking drunk, it’s an even sadder commentary that it’s seemingly even more dangerous to walk sober than to drive sober.