“I forgot” may (sometimes) be a credible excuse for breaking the speed limit

When someone breaks the speed limit, we tend to explain it away as recklessness, machismo, or impatience. But new research led by Vanessa Bowden at the University of Western Australia, suggests that problems in memory, not temperament, may often be the culprit. According to a recent study in the Journal of Experimental Psychology, traffic stops and other interruptions can disrupt our ability to keep track of recent changes to the speed limit. But the research doesn't entirely let us off the hook: when waiting at a stop, we can reduce these interfering effects by making sure we keep our attention on the road.

The research involved around 100 students at the University of Western Australia, mostly younger men, which is the demographic most likely to be involved in speeding-related accidents. Participants sat in a simulated driving cockpit with steering wheel, pedals and windscreen projecting a virtual road along which they had to drive for 15 kilometres without speeding, but also without dropping too far below the limit. In the first study, the limit was mostly a brisk 70km/hr, but it periodically decreased to 40km/hr for short sections of the road.

Five seconds after entering half of the 40 zones, participants were confronted with a red traffic signal, after which they were three times more likely to go on to speed in the zone – defined as five km/hr or more over the limit – than they were in the other half of the 40 zones, which had no interruption. In the rarer cases of speeding in the uninterrupted zones, the average top speed was just 47km/hr; a breach, but not a flagrant one. But the interrupted drivers reached an average top speed of 60km/hr, closer to the old limit than the current one. Bowden and her colleagues argue that this suggests that the interrupted drivers were frequently forgetting the new lower limit was in place.

These findings are consistent with similar evidence from real traffic data, which has also been interpreted in terms of drivers' disrupted memory. But an alternative explanation is simply that people find traffic stops frustrating and speed to make up for lost time. With no real-life appointments at stake, the current simulation research makes this seem less plausible. But Bowden's next experiment went one better, by flipping the zones so the default limit was 40km/hr and the exceptions were 70km/hr. They found that when a new 70 zone was interrupted by a red traffic light, participants tended to progress with an inappropriately slow speed, not faster: a quarter of interrupted 70 zones involved under-speeding, with participants driving on average at 50km/hr (in contrast, under-speeding, even merely 5km/hr under the limit, almost never occurred in uninterrupted zones). This is strong evidence that participants aren't hurrying after a light, but simply misremembering the new limit.

Other distractions can compound the problem. When the researchers gave participants a challenging task to do while waiting at a red light in a new speed zone (judging repeatedly whether the newest letter in series was earlier in the alphabet than the previous one; a real-life parallel might be using a red light stop to reprogramme your satnav), they found that this led to more instances of what they called "corrected speeding" where the participant shot off and immediately exceeded the limit, but caught themselves later in the zone. With a distracting task, this occurred in one in ten zones, compared to one in a hundred without a distracting task.

The researchers suggest that improving road safety is not only about changing attitudes but also about considering drivers' cognitive limitations, which gives some credence to the pleas of ignorance made by 20 to 50 per cent of drivers fined for speeding. Planners should think carefully about how speed changes intersect with focus-grabbing interruptions. And we should be aware that a stop in traffic isn't always an opportunity to daydream, change the radio, or fiddle with the satnav. The road is a complicated place and deserves our full attention.