In the preceding paper, we investigated driver's ability to recognize Head-up Display (HUD) images and the forward view comparing to Head-Down Displays (HDDs) by in-laboratory experiments. Recognizability of HUD images can be enhanced by setting the distance between driver's eyes and displayed images long, though the recognizability of the forward view does not depend upon the distance.
In this paper to verify the results, we have carried out two kinds of experiments using HUDs and a HDD installed in an experimental vehicle under actual running conditions. One is sensory evaluations of HUDs, and the other is measurement of eye fixation time for recognizing displayed images.
In recognizing displayed images and the forward view, the results of both the in-laboratory experiments and the actual vehicle experiments well match with each other.
HUDs can shorten the fixation time under all running conditions. For the recognizability of the HUD images, the increase in distance to the images improves the driver's recognizability. This is also true under actual running conditions, however, when the vehicle runs in city roads at low speed, the effect of moving vehicle brings in new factor that should be taken into account for optimizing the distance. The performance of HUDs that is related to this factor deteriorates when the distance increases.
It is concluded that the new factor should be traded off with the recognizability of HUD images to optimize the distance.