Auto safety and human adaptation

Issues in Science and Technology, Winter 2000/2001 by Smiley, Alison

Further research is required on other potential changes resulting from the use of navigation systems. There may be more driving by drivers unfamiliar with routes. There may be less attention to the road ahead, resulting in poorer detection of hazards. The overall safety effect will depend on the tradeoff between fewer lost and distracted drivers relative to greater exposure of unfamiliar drivers. It will also depend on the tradeoff between reduced attention required to the road ahead because of the navigation task being aided and greater demands required inside the vehicle.

Avoiding collisions

If the task is changed, drivers will modify their behavior. The task of designers and researchers is to ensure that the design encourages optimal modification. This is done by considering the likely changes in strategy and by modifying the design to ensure that the resulting behavior is appropriate to the design goal of increased safety.

A good example of this approach is a 1997 study by Weil Janssen of TNO Human Factors Research Institute, Soesterberg, the Netherlands, and Hugh Thomas of Bristol Aerospace, Bristol, United Kingdom. Performance was measured for three types of collision avoidance systems: driver's braking distance shown by a horizontal red line projected onto the windshield with a heads-up display; drivers warned through accelerator pedal resistance when the time to collision to another vehicle was less than 4 seconds; and drivers warned, as above, either when time to collision was less than 4 seconds or when the time headway to the car in front was less than 1 second

These three systems were compared with the use of a driving simulator. Vehicles ahead were presented with an initial headway of seven seconds. A variety of closing speeds were used, ranging from 10 to 40 kilometers per hour. In a quarter of the scenarios, the vehicle ahead of the driver braked, creating an emergency situation. Frequent but irregular oncoming traffic made passing difficult. The results showed that only the second collision avoidance system, which warned the driver of less than four seconds time to collision, provided a safety benefit. It reduced the percentage of time that the headway was less than one second, without increasing average speed. In simulated fog conditions, the heads-up display that showed braking distance significantly decreased driver safety by increasing short headways relative to when drivers had no collision warning system.

Based on a purely mechanistic analysis, one would expect the third system to be better than the second. However, the results showed that adding a simple one-second headway trigger criterion to four seconds time-to-collision criterion significantly worsened driver safety by increasing the proportion of short (less than one second) headways and the average speed. Because there were two distinct criteria, drivers may have found it more difficult to understand how the system was operating. It is sobering to remember that, in one of the first accidents involving an anti-lock braking system, a police officer in a high-speed chase responded to the unfamiliar vibration of the anti-lock brake by releasing his foot. In short, the driver's understanding of how the device operates is an important issue that has received little attention to date.