Differences in gap acceptance for approaching cars and motorcycles at junctions: What causes the size-arrival effect?

This study investigated whether the size-arrival effect for approaching vehicles, whereby people judge that approaching motorcycles will arrive later than approaching cars, is more likely to be due to overestimating the distance available in front of motorcycles or underestimating the speed of approaching motorcycles relative to cars. Approaching vehicles at junctions (cars and motorcycles) were shown in a series of video clips (speed and distance information was provided) and photographs (only distance information was provided). Drivers’ judgments about whether it was safe to pull out was investigated. The vehicle effect arose only in the video condition when vehicles were presented at a far distance. It was concluded that drivers’ error in judgment is likely to be due either to the miss-estimation of the speed of approaching motorcycles or drivers making judgments based on the rate of optical expansion, rather than direct misperceptions of distance.