The transfer of technology from tactical aviation to surface transportation is taking place at an astonishing rate. The types of technology now being integrated into automobiles include proximity warning systems; vehicle monitor and control systems; on-board navigation systems with route optimization algorithms, cartographic databases, electronic moving maps, and real-time communication from Global Positioning System (GPS) satellites; and on-board sensors to produce video imagery (Intelligent Transportation Society of America, 1994). The anticipated benefits of transferring these various types of defense technology to automobiles are intimately tied to the manner in which the driver may receive the information generated by them.
Much of this information is likely to be presented on head-up displays (HUDs), yet another piece of aviation technology. Because HUDs carry their own unique benefits and risks, in this article I attempt to separate the medium from the message, the display from the information. This critical review examines the evidence - or lack of evidence - of the benefits and risks associated with automotive HUDs. Particular emphasis is given to the manner in which the evidence relates to two issues that may well create serious safety hazards and which have been largely neglected in the automotive context: (a) the interaction between HUD focal distance and the perception of outside objects and (b) the capture of visual attention by HUD-projected virtual imagery.
The most thorough treatment of HUD design issues is Weintraub and Ensing's Human Factors Issues in Head-Up Display Design: The Book of HUD (1992). This compendious work provides an excellent background for the topics discussed here, even though fewer than two pages are devoted to automotive HUDs in a book almost entirely concerned with aviation. A report dealing specifically with automotive HUD research (Gish & Staplin, 1995) will provide an important resource to researchers engaged in the automotive HUD debate. Readers are encouraged to consult both texts.
The benefits expected from using HUDs derive from their two most salient features: the head-up location and a focal distance that is farther than the conventional instrument panel or dashboard. The head-up location is expected to permit drivers to keep their eyes on the road more than is possible with dashboard-mounted displays, which should result in better and faster detection of outside objects and events (an issue discussed at length in this article). A related benefit is that drivers will need less time to obtain information from a display that is head-up than from one that is head-down. Kiefer (1991) reported speedometer scan savings of 144 ms, though Gish and Staplin (1995) caution that this effect may not generalize to higher-workload conditions. Okabayashi et al. (1989) also reported lower recognition error rates and times for HUD imagery. The time advantage, however, diminished on curves - a finding that may substantiate Gish and Staplin's criticism that HUD advantages may be linked to low workload.
The only detriment so far attributed to the head-up location is the possibility that HUD imagery could obscure the driver's vision of outside objects (Okabayashi, Sakata, & Hatada, 1991), but this would be an issue only for imagery that directly overlays the forward scene.
The main benefit expected to accrue from the HUD's longer focal distance is a decrease in the accommodative shift (the distance that the eyes need to change focus between an outside object and a display). It is expected that the principal beneficiaries of the shorter focal transition will be the elderly, given their restricted accommodative range. Two studies, discussed later, provide evidence supporting this benefit to older drivers.
Given these and other attractive attributes, work has been proceeding to resolve the numerous display design issues involved in putting HUDs into automobiles - for instance, location, brightness, color, format, size, weight, and cost (see, e. …