Rearview Mirror Reflectivity and the Quality of Distance Information Available to Drivers

In two experiments, we examined the possibility that rearview mirror reflectivity influences drivers' perceptions of the distance to following vehicles. In the first experiment, subjects made magnitude estimates of the distance to a vehicle seen in a variable-reflectance rearview mirror. Reflectivity had a significant effect on the central tendency of subjects' judgments: distance estimates were greater when reflectivity was lower. There was no effect of reflectivity on the variability of judgments. In the second experiment, subjects were required to decide, under time pressure, whether a vehicle viewed in a variable-reflectance rearview mirror had been displaced toward them or away from them when they were shown two views of the vehicle in quick succession. We measured the speed and accuracy of their responses. Mirror reflectivity did not affect speed or accuracy, but it did cause a bias in the type of errors that subjects made. With lower reflectivity, there was an increase in errors in which displacements that were actually nearer were judged to be farther, and an approximately compensating decrease in errors in which displacements that were actually farther were judged to be nearer.

We interpret these results in terms of their implications for optimal reflectivity levels of rearview mirrors. The findings that reflectivity did not affect variability of magnitude estimates, reaction time for distance discrimination, or accuracy of distance discrimination suggest that the quality of distance information should not be considered a major factor in determining optimal reflectivity levels. However, these results should be interpreted within the limits of the methods of this study. Furthermore, the effect of reflectivity on the central tendency of magnitude estimates of distance should be explored further.