contrast of any visible light that may be seen through the HUD with the naked eye ( Weintraub & Ensing, 1992). The development trend to increase thermal image luminance ( Coonrod, 1983) would only serve to exacerbate this problem. A central high luminance source can also increase the adaptation level of the eye ( Howarth, 1990), decreasing visual sensitivity to lower luminance low contrast objects in the periphery, the very type of object (e.g., pedestrians) critical for a driver to detect.
Any or all of the aforementioned realistic VES technological limitations could increase the demands on driver attention in the real traffic situation or reduce the detectability of low contrast objects in the peripheral field of view, the practical effects of which may be to inhibit cognitive switching of attention between the VES image and scene even further than that demonstrated by this study.
Given the importance of peripheral vision in driving, there is a clear need for further research to assess the safety implications of VES-induced perceptual tunneling. Further research is recommended to determine if the performance decrement revealed in this study is affected by driver age, experience, visual function, training, or experience, each of which has been shown to have an effect on normal driver visual behavior ( Mourant & Rockwell, 1972; Shinar, McDowell, Rackoff, & Rockwell, 1978). The effects on peripheral visual performance under varying environmental and visibility conditions should also be examined because this study was limited to examination of ambient illumination only.
A number of VES variables also need to be evaluated in terms of their impact on attentional resources. These may include HUD image position, size, shape, collimation and luminance, thermal image-processing options, degrees of image disparity, and interactions with typical sources of illumination in the driving environment, each of which alone may affect driver performance with VES technologies.
Finally, more realistic simulation of the driving task, HUD attributes, environmental conditions, and presentation of potential obstacles is needed to permit more accurate generalization to the real driving environment.
We would like to thank B. Laffoley of Pilkington p.l.c., P. Marsh, and K. Cooper of HUSAT for their contribution in producing stimulus material, editing, and analysis software.