The distortion is most pronounced for drivers who are going much faster or slower than average, Bryan Dawson of Union University in Jackson, Tennessee told the Joint Mathematics Meetings in San Diego, California, last week.
The illusion happens because drivers can only gauge the speed of the cars around them, not all the cars on the motorway, Dawson says. A driver going considerably faster than average will overtake many slow cars but will only be overtaken by a few fast cars, making the average speed seem lower than it really is. Conversely, a slow driver will feel that a deceptively high proportion of the cars are driving fast.
In a model where the speeds of the cars were distributed in a smooth bell curve around an average of 68 miles per hour, Dawson found that a driver going at 65 miles per hour would think the average speed is over 70.
The illusion should be most pronounced and happen most quickly in heavy traffic because drivers pass and are passed by more other cars.
On congested roads, the skewed perception could make the brake and accelerator seem twice as sensitive as usual for a car driving at about average speed, Dawson suggests. "Maybe that is one of the reasons driving in heavy traffic is so nerve-racking," he says. "Not only do you have to navigate around lots of cars, but each time you brake or accelerate your perception of reality changes."
Author: Erica Klarreich
New Scientist issue: 19th January 2002
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