In the United States, rear-end collisions represent approximately 30% of all car crashes on public roads (National Highway Traffic Safety Administration, 1999). Two major causes of such accidents are driver inattention and failure to maintain the proper distance from the lead car (Dingus, Jahns, Horowitz, & Knipling, 1998). Therefore, a device that can both alert an inattentive driver to an impending crash and help the driver maintain an appropriate distance from the lead car could be an important safety tool.
Two measures are commonly used for converting the distance between vehicles traveling in the same direction into a unit of time. One is time to collision (TTC), or the time it will take for two cars at their present speeds to collide. The second measure, the one used in this study, is temporal headway (TH), the time it will take for the following car to reach the position of the lead car. Although drivers are taught to maintain a safe headway of 2 s or more from the lead car, and drivers' handbooks provide information about driver reaction time and stopping distance (e.g., Maryland Drivers' Handbook, 1998; National Safety Council, 1992), in real driving situations headways of 1 s or less are typical of fast rush hour traffic (e.g., Chen, 1996; Evans & Wasielewski, 1983).
Consequently, past research carried out by Taieb and Shinar (2001) showed that drivers tend to overestimate their headways despite years of driving experience. Evans (1991) suggested three reasons for this risky behavior. First, on the highway, drivers rely on the fact that sudden deceleration by the lead car rarely occurs. Second, they view the lead vehicle speed as a constant, so that if they match its speed, an accident will not occur. Finally, past experience has reinforced such short headway. One reason that drivers tend to misjudge other vehicles' speeds may be related to the difficulty in perceiving external objects' movement in relation to one's own movement (Rumar, 1990).
There have been numerous instances of drivers reporting that they simply did not see the other vehicle until it was too late, commonly referred to as "looking but not seeing" (Stone, 1977; Treat et al., 1977), the cause of which is probably error in perceptual or cognitive recognition (Crundall & Underwood, 1997; Rumar, 1990; Stone, 1977).
Driver errors in headway judgment and in the detection of other vehicles' movement lead to the possibility of using technological devices both as a way to educate the driver and as a means to alert the driver to situations that he or she may not have perceived. Such a device measures the TH and sounds a warning beep when the headway to the lead car is shorter than a predefined threshold.
There are two human factors issues in the implementation of an in-vehicle collision avoidance warning system (IVCAWS). The first is the interface to use in relaying the information from the automatic system to the driver. Some studies compared different methods of warning (visual, auditory, and combinations of the two) and found that most were effective to some degree (Dingus et al., 1997), with an auditory tone being the most effective interface (Hirst & Graham, 1997; Maltz, Aminov, Aharonov, & Shinar, 1999).
The second issue is how to analyze the interaction between the driver and the automated warning system when both are capable of error. Sorkin and Woods (1985) recommended that analysis of human performance with an automated aid should be considered a combination of the performance of the automatic system and of the human's subsequent behavior. The automatic system's performance is defined by its probability of detection (of an unsafe headway) and by its probability of a false alarm. The human's behavior is based both on his or her own processing of the event and on the information provided by the automatic system. …