Crash! Collisions in the Mind's Eye

Is it possible that automobile drivers are routinely misjudging motorcycles' distance and speed, and that this misperception is somehow related to the motorcycles' size?
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My son was involved in a serious motorcycle accident some months ago. He was driving on a major avenue in Washington, D.C., going the posted speed, when a taxi pulled out from a side road, directly into his path. My son hit the brakes, but the cab was too close to avoid, so he deliberately took a spill. Both he and the bike slid under the cab, which mercifully stopped, inches before running over him.

He was injured and shaken, the bike was totaled, and I was both relieved and angry. It's not clear whether the cabbie misjudged my son's speed or his distance from the intersection or was simply not paying attention. At the time, I didn't much care about the particulars. All I knew was that this cabbie's reckless driving was a threat, and I took it personally.

I've eased up -- just a bit -- in part because I've since learned a little about motorcycle accidents of this kind. They're quite common. Indeed, most crashes involving a motorcycle and a car occur when the driver of the car violates the motorcyclist's right-of-way, not the other way around. The car encroaches on the bike's path -- almost exactly as it happened with my son -- too late to avoid a collision. Many cyclists aren't so lucky.

Psychological scientists are very interested in this pattern of collisions, as it turns out. Patricia DeLucia, a perception expert at Texas Tech University, wondered if there might be a fundamental perception problem at the root of these crashes. Is it possible that automobile drivers are routinely misjudging motorcycles' distance and speed, and that this misperception is somehow related to the motorcycles' size?

DeLucia doesn't study traffic accidents or motorcycles exactly, but she does study collisions. She studies collisions in the broadest sense, both those we want to avoid -- like traffic accidents -- and the ones we choose -- like when a linebacker sacks a quarterback. Her work focuses on how we judge size and motion in order to calculate and anticipate these crashes. Her findings suggest that we're not very rational or objective in making these judgments.

Here's why. When the mind sees an approaching vehicle, and wants to estimate the time to collision, it relies on at least two sources of information to make the calculation. The first is an accurate one-to-one representation of the visual field on the retina; it's an objective calculation based on an object's size at a given instant and its rate of expansion. Theoretically, this is all the mind should need to come up with the collision time -- and get out of the way -- but it is not the mind's only source of information. The mind also uses various depth cues as a mental shortcut: For example, there is a cognitive rule of thumb that says bigger is closer, which is often but not always true.

One would think that, given both kinds of information to work with, the mind would opt for the more reliable and objective kind over the more intuitive guesstimates. But DeLucia's lab work is showing that this is not so. Indeed, it appears that we make time-to-collision judgments based on less reliable depth cues even when more objective information is available. Perception researchers call this irrational kind of thinking the "size-arrival effect."

DeLucia has run many experiments showing the potency of this cognitive bias. In a typical study, volunteers see two computer-generated spheres of different size approaching at the same speed. The smaller object is closer to the eye, so it's possible to know for a fact that it will arrive first. Yet volunteers consistently say that the larger object is going to arrive first, suggesting that the mind privileges the less reliable, more intuitive size cue over objective calculation. DeLucia and other scientists have demonstrated this robust size-arrival effect in many variations of this study, including one in which the observers had to "jump" to avoid being hit. If they were about to collide with a large object, they jumped earlier.

DeLucia writes about the practical implications of this lab work in a paper that will appear in a future issue of the journal Current Directions in Psychological Science. Approaching vehicles are just like the computer-generated spheres, and are most likely misperceived in the same way. So it's likely that drivers are risking collisions by turning late in front of small oncoming vehicles, like motorcycles, which appear farther away than they really are. Drivers are probably estimating, inaccurately, that they have more time than they actually do. In fact, DeLucia describes more realistic studies simulating actual traffic intersections, which show that motorcycles are more likely to be misjudged than are cars or vans.

I wasn't crazy about my son buying a motorcycle in the first place, and I told him at the time that my feelings had nothing to do with him or his driving. I knew him to be conscientious, but I did not know the same of other drivers. But I was thinking more about attitude, what I saw as a casual disregard for the rights of cyclists and bikers, who are so vulnerable to injury. Little did I know that this recklessness is even more deep-rooted than I suspected, right down in the neurons of the mind's eye.

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