A Field Study of Distance Perception with Large-Radius Convex Rearview Mirrors

One of the primary reasons that FMVSS 111 currently requires flat rearview mirrors as original equipment on the driver's side of passenger cars is a concern that convex mirrors might reduce safety by causing drivers to overestimate the distances to following vehicles. Several previous studies of the effects of convex rearview mirrors have indicated that they do cause overestimations of distance, but of much lower magnitude than would be expected based on the mirrors' levels of image minification and the resulting visual angles experienced by drivers. Previous studies have investigated mirrors with radiuses of curvature up to 2000 mm. The present empirical study was designed to investigate the effects of mirrors with larger radiuses (up to 8900 mm). Such results are of interest because of the possible use of large radiuses in some aspheric mirror designs, and because of the information they provide about the basic mechanisms by which convex mirrors affect distance perception.

Subjects' distance perceptions for objects seen in large-radius rearview mirrors were measured by magnitude estimation in a static field setting. The results indicate that overestimation of distance continues to decrease as mirror radius increases beyond 2000 mm, and that the overestimation continues to be substantially lower than would be predicted from a model based on image minification and reduction of visual angle. However, even at the longest radius examined in this experiment (8900 mm) the overestimation of distance (8%) is not small enough to be dismissed definitively as trivial. Because various learning effects and changes in driver strategy may compensate for the distortion of distance perception, this does not necessarily mean that convex mirrors of any radius are unsafe. But it suggests that, even for convex mirrors with very long radiuses, the gain in quantity of field of view provided by the convexity comes with a nontrivial cost in quality of field of view, and that the tradeoff between these two characteristics must still be considered in designing optimal mirrors.