Over time, our brains have become expert detectors of dangerous environmental cues. While many of us don’t need our vision to scan the tall grass for a pair of predatory eyes, we do face numerous hazards over the course of the day, many of which we may take for granted. Driving is perhaps the most common, yet a new study suggests it’s the "element of surprise" that makes motorcycles such a danger to other drivers.

There’s a term in psychology known as the "prevalence effect." Basically, it refers to the brain’s ability (or inability) to marshal a person’s visual system properly. Things that we see often have a high prevalence. People, buildings, cars — we get used to seeing them, so we grow to expect them. But for every person, building, and car, there is an outlier — something that catches us off guard. Sometimes this is harmless. But catch a motorcycle weaving into your lane too late, and you may have a high-speed collision on your hands.

Researchers from Monash University sought to better understand the low-prevalence effect — what happens when the brain fails to catch something, or catches it too late, because it wasn’t prepared to see it. Research has already established the effect’s presence in certain areas. Airport security, for instance, must constantly fight the low-prevalence effect when scanning for dangerous items. Before terrorists started making liquid bombs inside of shampoo bottles, security guards rarely noticed them. But now the threat is greater, so personnel scan for them the same way they scan for knives and guns.

To carry out their study, the research team recruited 40 adult drivers to use a simulator that would throw two types of hazards at them, motorcycles and buses, while they were driving. They were curious whether the drivers would have an easier time noticing the motorcycles and buses when there were more or fewer of the other kind of vehicle on the road. One half of the subjects drove with a greater number of motorcycles and fewer buses, and the other group received the opposite.

In both groups, the vehicle that subjects detected more quickly, most often, and from a greater distance, was the one that appeared in greater numbers. Because the subjects saw one type faster than another, their ability to detect the low-prevalence vehicle necessarily suffered. To put it quantitatively, those who saw more motorcycles saw them an average of three seconds earlier, while the bus group saw those vehicles 4.4 seconds earlier when there were more of them.

“Drivers have more difficulty detecting vehicles and hazards that are rare, compared to objects that they see frequently,” said study leader Vanessa Beanland in a statement, adding that the ability to accurately perform visual searches is crucial to ensuring safe driving and avoiding collisions.

Ameliorating this problem isn’t the easiest. In an ideal world, there would be an equal number of each type of vehicle on the roadways. But since this isn’t feasible, a more workable solution would be to generate awareness among drivers, letting people know that their vision is controlled, in part, by their surroundings. Drivers are instructed since their sophomore year of high school to scan the road, though probably less often for specific types of vehicles just because there are fewer of them. But according to Beanland and her colleagues, that is sound advice.

Source: Beanland V, Lenné M, Underwood G. Safety in numbers: Target prevalence affects the detection of vehicles during simulated driving. Attention, Perception, & Psychophysics. 2014.