IF YOU constantly feel all the cars on the motorway are driving too fast or
too slow, don鈥檛 worry, it鈥檚 probably an illusion. The speed at which you drive
seems to skew your perception of the prevailing speed of traffic, even if you
accurately judge the speed of each individual car.
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.
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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. 鈥淢aybe that is one of the reasons driving in heavy
traffic is so nerve-racking,鈥� he says. 鈥淣ot only do you have to navigate around
lots of cars, but each time you brake or accelerate your perception of reality
肠丑补苍驳别蝉.鈥�