The U.S. currently requires that reai-view mirrors installed as original equipment in the center and driver-side positions be flat. There has recently been interest in using nonplanar mirrors in those positions, including possibly mirrors with large radii (over 2 m). This has provided additional motivation to understand the effects of mirror curvature on drivers' perceptions of distance and speed. This paper addresses this issue by (1) reviewing the concepts from perceptual theory that are most relevant to predicting and understanding how drivers judge distance in nonplanar rearview mirrors, and (2) reviewing the past empirical studies that have manipulated mirror curvature and measured some aspect of distance perception.
The effects of mirror curvature on cues for distance perception do not lead to simple predictions. The most obvious model is one based on visual angle, according to which convex mirrors should generally lead to overestimation of distances. But convex mirrors affect other perceptual cues (including vergence and accommodation) in ways that lead to predictions of underestimation.
Empirical investigations of the effects of mirror curvature have produced a strong consensus that convex mirrors cause overestimation of distance, but several factors can moderate or compensate for that effect. All quantitative studies of the effects of the radius of convex mirrors have demonstrated less overestimation of distance than predicted by the visual-angle model. Shorter-radius (more strongly curved) mirrors generally lead to greater overestimation of distance. Previous studies have examined the effects of mirror radius up to 2 m. There is strong evidence that 2-m mirrors still cause substantial overestimation, and little indication that reductions in overestimation have asymptoted at that radius.
New empirical efforts to study the effects of larger-radius mirrors, beyond 2 m, could both contribute to basic understanding of how minor curvature affects distance perception, and provide practical information about the possible benefits of using large-radius rearview mirrors.