Are Those other Drivers Really Going Faster?

Motor-vehicle travel is a mixed blessing for modern times. During the average day in the United States, for example, about 100 people step into avehicle and do not emerge alive according to data from the 1996 Statistical Abstract of the United States, published by the Bureau of the Census. Crashes are especially poignant if they kill healthy people who otherwise might have led long and productive 1ives.A 1957 New England]ournu1 OfMedicine study found that crashes are usually (>90%) attributed to driver error rather than failures in the vehicle or roadway. The most important factor in driver error is alcohol, contributing to about 40% of fatal collisions in 1994 according to a National Highway Traffic Safety Administration report. The other factors causing driver error are not completely known. A better understanding of such errors might allow people to benefit from motor vehicle travel at a lower personal risk. One potential driver error is an inappropriate lane change, a vehicle maneuver that may have substantial risks for several reasons. First, it causes the individual to straddle traffic flows and be exposed to two streams of vehicles. Second, it requires the driver to make rapid judgments about sufficient spacing. Third, it increases the hazard related to other vehicles approaching along the driver's blind spot. Fourth, it disrupts the traffic pattern for following vehicles. The overall risks associated with each lane change are uncertain because the amount of normal driving spent making lane changes is not known with precision; however, rough estimates in an Ontario Ministry of Transportation report suggest about a threefold relative risk if less than 1% of normal driving involves a lane change. We wondered whether people can accurately judge if they are in a lane that is slower than the next lane on a congested roadway. Mistaken impressions, for example, might cause a driver to incorrectly think the next lane is faster and motivate a needless lane change. Perhaps errors in judgment produce a systematic bias and create an illusion that the next lane is generally moving faster, even if all lanes have the same average speed. One basis for such error is if drivers expect that they should spend equal amounts of time passing and being overtaken. We have shown that such an expectation is mistaken when time is measured by discrete intervals. We recently popularized this finding in an exceedingly short paper