Risk homeostasis

I recently finished reading Jeff Speck’s The Walkable City: How Downtown Can Save America One Step at a Time. I like my location in part because I can walk to the post office, a grocery store, my kids’ schools, and the library if I want. I usually do stop by the post office on my runs, and I enjoy that feeling that the run has a purpose.

But anyway, one of the more interesting concepts in the book, which I knew about but didn’t know had a name, is risk homeostasis.

Risk homeostasis recognizes that human behavior is not a constant. Behavior is highly variable based on incentives and social cues. What is constant is that people have a certain feeling of safety they like to maintain. This may be slightly different for every person, but the comfortable level of risk is the constant, not behavior.

Consequently, when you make traffic lanes wider, people feel more safe. So they speed up. This makes them less safe (crashes at high speeds are more deadly than at lower speeds) but they feel equally safe. If you narrow the lanes, they feel less safe (that oncoming car or mailbox could hit me!). So they slow down. But, of course, this makes them safer.

The trouble, Speck notes, is that traffic engineering has, until recently, not taken this concept into account. New road construction often requires intersections to be at right angles, and with nothing blocking sight lines in any direction. The perception is that a 5-way intersection, or an angled intersection, or one with numerous trees on the corners, is not safe because you can’t see. But because of the concept of risk homeostasis, people feel that the right-angle intersections are safe and so they are more likely to speed through on yellow/red lights, slow but not really stop at stop signs, not look all directions, etc. This can make such intersections more deadly than “unsafe” feeling intersections.

I live in a pretty old area where some streets have been around since before modern traffic engineering. We have one intersection near us that has four streets coming in at 45/135 degree angles. They meet at the top of a hill, and because there’s a cut-through on one of the acute angles, and the township put a tiny little park on the space left in the intersection adjacent to the cut out, you really can’t see a darn thing. Before reading The Walkable City, I thought that intersection was unsafe. But now I notice that I inch across it at 5 m.p.h. Most likely, other drivers feel the same way and, in the interest of self-preservation, are also inching across. If we’re all going 5 m.p.h., we’re going to be able to stop long before a crash would occur. So we’re actually pretty safe. 

Do you have any examples of risk homeostasis at work on streets near you?

4 thoughts on “Risk homeostasis

  1. I dunno, last car accident I was in, I was inching carefully forward at a blind intersection and got hit by a truck. And there were pieces of all sorts of other cars that had been hit at that intersection. There’s probably some sort of U-shaped curve in which truly blind intersections really are dangerous, and intersections that have almost no cars ever probably are pretty safe to breeze through, comparatively.

  2. While driving between Ontario and Bend, Oregon on Highway 20, I went for 12 minutes without seeing a car from the opposite direction and for 50 minutes without seeing a car on the same side of the road as I was.

    Then I saw a cow on the road and the sheriff for the eastern half of the state valiantly trying to get the cow OFF the road.

  3. I can think of whole counties and countries that may be good examples of this. I live in the LA area, and I’ve heard before that our crazy traffic makes us safer — can’t get up to speed for fatal accidents. Obviously that isn’t always the case, there are places and times that are much lower traffic, but as a general trend.

    Also I spent a couple months once on an internship in southern Mexico, living with an American family most of that time. I was told that, scary as the driving might seem (WAY scarier than LA), the lack of the American value for predictability made them safer, because they didn’t expect anyone to drive predictably. Oh, and I remember one time we were warned about a particularly bad road; we thought it would have lots of potholes and such. No, it was “bad” for exactly the opposite reason — a nice paved mountain road that people drove way too fast on. Very dangerous.

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