But Officer, It Was the Fog

Thick fog poses an obvious traffic hazard--you can't see very far. Now scientists have identified a surprising danger of driving in pea soup: The lack of contrast makes high speeds seem slow. This visual illusion, reported in this week's Nature, suggests that more-prominently placed speedometers might prevent some accidents due to excessive speed in bad weather.

Perception of speed depends on the amount of contrast between objects and the background, say, a fence against a field. As contrast decreases, objects appear to move more slowly. Robert Snowden, a psychologist at the University of Wales in Cardiff, U.K., and colleagues wanted to know how this change in perception applies to driving in fog or other poor visibility conditions.

Snowden's team used a virtual reality device to simulate movement in clear, misty, or foggy conditions. First they monitored the reactions of five people watching simple moving patterns, such as stripes and dots, on a clear day. Then they gradually ratcheted down the contrast of the patterns by adding opaque screens; as visibility was reduced, the subjects consistently thought the patterns were moving slower, when in fact they were not. Next, the researchers trained nine people to drive a simulated vehicle along a winding road in good weather, then started mixing in some bad weather. The subjects were told to maintain a certain speed throughout the simulation, but they were unable to. "When they were asked to drive 50 miles an hour and it was sunny, they drove about 50 miles an hour; when it was foggy, they drove at about 65 to 70 miles an hour," says Snowden.

So don't always blame foggy traffic accidents on reckless drivers, says Snowden: Some may simply be confused about their speed. A speaking speedometer or one located on the windshield could prevent this confusion, he says.