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New developments in the use of two-dimensional displays to supplement driver vision have made it more important to understand the roles that various distance cues play in driver perception of distance in more conventional ways of viewing the road, including direct vision and viewing through rearview mirrors. The current study was designed to investigate the role of binocular distance cues for perception of distance in rearview mirrors. In a field experiment, we obtained data to estimate the importance of binocular cues for distance judgments under conditions representative of real-world traffic. The results indicate that, although binocular cues are potentially available to drivers, these cues probably play little or no role in distance judgments in rearview mirrors in normal driving situations.

The importance of eye-to-display distance for distance perception in rear vision may depend on the type of display. At least in terms of its influence on the effective magnification of images, eye-to-display distance is almost irrelevant for flat rearview mirrors, but it is important for convex rearview mirrors and for other displays, such as video displays, that create images closer to the driver than the actual objects of interest. In the experiment we report here, we investigate the influence of eye-to-display distance on distance perception with both flat rearview mirrors and camera-based video displays. The results indicate that a simple model of distance perception based on the visual angles of images is not very successful. Visual angles may be important, but it appears that relationships between images of distant objects and the frames of the displays are also important. Further work is needed to fully understand how drivers might judge distance in camera-based displays.

The increasing availability of camera-based displays for indirect vision in vehicles is providing new opportunities to supplement drivers' direct views of the roadway and surrounding traffic, and is also raising new issues about how drivers perceive the positions and movements of surrounding vehicles. We recently reported evidence that drivers' perception of the distance to rearward vehicles seen in camera-based displays is affected not only by the visual angles subtended by the images of those vehicles, but also by the sizes of those images relative to the sizes of the displays within which they are seen (an influence that we have referred to as a framing effect). There was also evidence for a similar, but weaker, effect with rearview mirrors.

Advances in camera and display technology have increased interest in using camera-based systems for all rear-vision functions. The flexibility of camera-based systems is unprecedented, and raises the possibility of providing drivers with fields of view that are very different from, and potentially much better than, those of conventional rearview mirrors. Current fields of view are based on a combination of driver needs and the practical constraints of mirror systems. In order to make the best use of the greater flexibility offered by cameras, a reassessment of drivers' needs for rear vision is needed. A full reassessment will require consideration of many factors. This paper offers a preliminary analysis of one of those factors: the visual workload involved in using rear-vision systems with single versus multiple displays.

Conventional brake lights require 250 msec to reach 90% intensity, thereby causing potentially important delays of warning information to following drivers. Several improvements are possible, including the use of LED displays. LED's, however, are more expensive than conventional incandescent bulbs and require redesign of lamp housings. As an alternative, we have designed a simple and relatively inexpensive circuit that produces a faster warning signal using a conventional bulb. We have evaluated the benefits of this device in a laboratory study that measured subjects' reaction times to the onset of brake lights in a simulated car-following situation. Our data indicate that the benefit of the device is on the order of 115 msec. For a vehicle traveling at 65 miles per hour, that benefit translates to a decrease in stopping distance of 11 feet.

The effects of image size on perceived distance have been of concern for convex rearview mirrors as well as camera-based rear vision systems. We suggest that the importance of image size is limited to cases-such as current rearview mirrors-in which the field of view is small. With larger, richer fields of view it is likely that other distance cues will dominate image size, thereby substantially diminishing the concern that distortions of size will result in distortions of distance perception. We report results from an experiment performed in a driving simulator, with static simulated rearward images, in which subjects were asked to make judgments about the distance to a rearward vehicle. The images showed a field of view substantially wider than provided by any of the individual rearview mirrors in current systems. The field of view was 38 degrees wide and was presented on displays that were either 16.7 or 8.5 degrees wide, thus minifying images by factors of 0.44 or 0.22.