Illusory size-speed bias: Could this help explain motorist collisions with railway trains and other large vehicles?

Collisions between motorcars and larger transport vehicles are over-represented in New Zealand’s serious crash statistics. These types of collisions include incidents between motor vehicles and trains at railway level crossing junctions, and collisions with vehicles such as long-haul trucks at T-intersections. Errors made in judging a large vehicle’s speed could, in part, be attributed to motorists being unknowingly subjected to a size-speed illusion. Previous research has found that humans perceive large objects (e.g. trains, trucks, buses) as moving more slowly than smaller objects (e.g. motorcars) travelling at the same speed. Generally these studies have involved participants viewing approaching stimuli from a stationary position. However, the reasons behind the size-speed illusion remains unknown, and research has not yet determined whether a motorist’s self-motion has a bearing on size/speed effects. To investigate these issues we tested observers’ relative speed estimation performance for a train and a car approaching at a range of speeds and distances, using psychophysical methods. An eye tracker was also used to provide information about where participants fixated and their pursuit velocities. The data show that participants significantly underestimated the speed of the train, compared to the car. A size-speed illusion seems to be operating in the case of the approaching train in our simulation and may also be a factor in other heavy duty vehicle collisions.