The recent discussion in the press of illusory speed bumps in Philadelphia reminded me of a conversation I had a while back with the Chicago Department of Transportation. For years, the curve at Oak Street on Lake Shore Drive has been something of a crash hot-spot. It’s within the typical engineering guidance for acceptable curves, but given that drivers on LSD tend to treat the Drive as an urban expressway rather than the boulevard it really should be, there’s been a consistent crash problem, particularly on rainy days and the like.

CDOT responded with a series of gradual alterations. They made the lane markings more distinct. They made the curve warning signs bigger. Then they added flashing lights. Drivers still weren’t getting the message. So CDOT got perceptual.

CDOT responded with what’s become known in the biz as an “illusory pavement marking,” in this case a set of transverse optical bars painted in the road that shrink as the driver gets closer to the curve. The idea is it makes them think they are going faster than they really are. This approach has been used at places like highway off-ramps, where drivers, their perceptive systems warped by highway speeds, often fail to sufficiently slow down.

It’s too soon to tell, but anecdotally CDOT has seen an improvement in behavior. One critique of such systems is that they rely on a novelty effect — then again, as Necker Cubes and the like show, we tend to fall for visual illusions even when we know they are illusions. There’s one argument in traffic engineering that uncertainty is a good thing — when the uncertainty in question forces drivers to act more cautiously in the face of that uncertainty, rather than being overwhelmed by it. Another argument in favor of the LSD bars comes from behavioral economists Cass Sunstein and Richard Thaler in their book Nudge. To them, the Lake Shore Drive bars are a form of “libertarian paternalism” that allows people to make their own decisions, but subtly suggests (more subtly than a speed limit sign and a curve warning, that is), through “choice architecture,” what the right course of action is. The idea is it’s more effective to make someone feel as if they’re going too fast, rather than trying to tell them so.

In the Philadelphia case, shown in the photo below, one of the main drawbacks is that people who live in the neighborhood or use the road regularly (and remember most crashes happen close to our homes) will become familiar with the visually confusing speed bumps. There’s other ways to tackle the problem, however. The road could be narrowed — a proven speed reducer — or, similarly, parking permitted on both sides (it’s unclear from the photo whether that’s the case). Different types of pavement treatments could be installed to break up the visual notion of the road as a straightaway. Most ambitiously, the yellow line could be removed. A number of studies have shown that, in the absence of a dividing line, speeds decrease, while distance between opposing traffic streams actually grows. The yellow line is a subtle signal to speed up — one’s territory is “safely” marked. Whether removing the line is more or less “safe” is a relative question; after all, the best safety measure for all involved, drivers and neighborhood residents, would be lower speeds.

This entry was posted on Monday, July 14th, 2008 at 2:17 pm and is filed under Drivers, Roads, Traffic Engineering, Traffic Psychology, Traffic Wonkery, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.