Innocent racers in the fog?
Thick fog obviously poses a traffic hazard due to the low visibility. Nevertheless, many cars drive too fast. Surprisingly, this misbehavior can be partly attributed to an optical illusion due to the low contrast of landscapes in fog. Prominently placed speedometers could help avoid speeding accidents in inclement weather. The perception of speed depends on the contrast between objects and the background, for example a fence in front of a meadow (Nature 2 April 1998). As contrast decreases, objects appear to move more slowly. Robert Snowden, a psychologist at the University of Wales in Cardiff, and his colleagues wanted to find out how this change in perception affects driving in fog or other low-visibility situations.
Snowden's team used a virtual reality producing device to simulate movement in clear, hazy or foggy conditions. First, they monitored the reactions of five people looking at simple movement patterns, such as streaks and dots, on a clear day. Then they gradually reduced the contrast of the patterns by introducing opaque screens into the experimental setup. As visibility decreased, subjects consistently believed that the patterns were moving more slowly, which in fact was not the case.
Next, the researchers trained nine people to drive a simulated car on a winding road in good weather, and then mixed in bad weather. The subjects were supposed to maintain a certain speed throughout the simulation, but they were unable to do so. "When they were asked to drive 75 km/h in sunshine, they drove about 75 km/h; in fog, they drove about 90 to 100 km/h," says Snowden.
So don't always blame a careless attitude for traffic accidents in the fog, he says. Some drivers are unable to correctly estimate their speed in such conditions. A talking speedometer, or one built into the windshield, could help avoid that confusion, Snowden believes.