The insurance industry has long said that punitive speed enforcement is essential for traffic safety. Strict punishment is needed, they insist, to prevent the intentional speeding of impatient motorists. In a study to be published in the Journal of Experimental Psychology: Applied, researchers from the University of Western Australia provide evidence they believe suggests exceeding the speed limit has more to do with unintentional memory lapses.

"Existing programs to reduce speeding have focused on enforcement and punitive measures to discourage drivers from choosing to exceed the speed limit," the study explains. "Critically however, twenty to fifty percent of drivers fined for speeding report that they were unaware of their actions until ticketed for the offense."

Professors Troy A. W. Visser and Shayne Loft and research associate Vanessa K. Bowden set up a series of driving simulation experiments to test their theory about the psychology of driving past a speed limit sign. Sixteen undergraduate students were placed in the simulators driving on a 70km/h (43 MPH) road that had the speed limit reduced to 40km/h (25 MPH) ten times along the way. In half of the speed reduction zones, the students were interrupted by a traffic light changing to red. As an incentive to obey the speed limit, the students were to be given a $5 bonus for keeping to the appropriate speed.

"Participants were nearly three times more likely to speed throughout the entire 40-zone after an interruption," the study found. "When speeding occurred in an interrupted 40-zone the average maximum speed reached was 59.7km/h, which suggests that participants were attempting to return to the previous but no longer relevant speed limit of 70km/h."

The next test examined driver response to the speed limit increasing from 40km/h (25 MPH) to 70km/h (43 MPH). The results showed many cases where, after being interrupted, the drivers did not speed up -- a phenomenon they called "under-speeding." This, the researchers suggest, is evidence drivers are not intentionally driving as fast as possible in all cases, as the insurance industry insists. Rather, they forget to adjust speed when distracted by, for example, a traffic light changing to red.

"In other words, we know that drivers will forget and go too fast, but will they also forget and go too slow?" the researchers asked. "Our results showed that this was the case, and forgetting was just as likely to cause under-speeding as it was over-speeding -- with 23 percent of interrupted drivers forgetting and traveling too slowly compared to one percent of uninterrupted drivers."

The final test distracted participants with a cognitively demanding task. The students were forced to listen to a series of letters being read aloud and indicate whether the current letter came before or after the one that preceded it. This task increased the likelihood of initial speeding by up to 11 percent.

"This finding supports the conclusion that prospective memory failure is involved, since participants initially sped when they had fewer cognitive resources available to retrieve the new reduced speed limit," the study concluded.