Academic journal article Human Factors

Minimum and Comfortable Driving Headways: Reality versus Perception

Academic journal article Human Factors

Minimum and Comfortable Driving Headways: Reality versus Perception

Article excerpt

A field study was conducted to evaluate drivers' actual headways in car-following situations, their relationship to the drivers' brake reaction times, and their relationship to the drivers' ability to estimate those headways using different metrics. Drivers were asked to maintain "minimum safe distance" and "comfortable, normal distance with no intention to pass" behind the car ahead. The lead car speeds varied from 50 to 100 km/hr. The results showed that under both sets of instructions, drivers adjusted their distance headways in relation to speed, maintaining constant time headways. A significant portion of the drivers maintained time headways that are considered unsafe in relation to drivers' reaction times. There was no significant relationship between the minimal headways maintained by the drivers and their brake reaction times under conditions of maximum attention and preparedness to apply brakes. Accuracy of spoken estimates of headways varied widely among the three measures used to report perceived h eadway; meters and car lengths yielded much lower estimates (and ones closer to the actual headways) than did seconds. The results have implications for headway perception, driving safety, driver education, and smart cruise-control design.


Approximately 25% of all road accidents are rear-end collisions. Studies that examine driver behavior under different traffic conditions show that one of the major contributors to these collisions is headway that is too short to allow the following driver to react appropriately to sudden braking by the lead vehicle (e.g., Knipling et al., 1993). All intelligent cruise control systems and other warning devices have to define "safe" headways. One method is to define it in terms of the vehicle's braking ability and the driver's reaction time (e.g., Dull & Peters, 1978). Another method is to use rules of thumb; for example, one car length for every 10 mph (16 km/hr; Ioannou, Chien, & Hause, 1992). The shortcoming of such approaches is that they do not necessarily reflect headways maintained in real-world driving, which are presumably based on drivers' perceptions of safe headways.

This study had four objectives, all related to drivers' actual and perceived

headways. The first objective was to determine drivers' choices of minimum safe headway and comfortable headway from the driver's perspective. Ohta (1993) found that drivers' actual headways range between the minimal headways perceived as safe and the headways at which drivers feel that "they are not too close and not too far" from the vehicle ahead. Somewhere within this range is the headway that the driver finds comfortable. In this study we evaluated a wider range of speeds than previously examined.

To determine liability for maintaining safe headway and to enforce safe headways, it is necessary to first examine whether the problem lies in the drivers' inability to correctly estimate distances to vehicles ahead, or whether the drivers knowingly violate the law. When the environment has enough cues for size and object constancy, the perceived distance is very similar to the physical distance. Yet under the dynamic conditions of driving, the task becomes more difficult, as the number and type of cues available for distance estimation change uncontrollably from moment to moment.

In many situations, people can make certain judgments even when there is no spoken access to visually processed information. There are also differences in accuracy and variance between verbal and nonverbal estimates (for a review, see Leibowitz, Guzy, Peterson, & Blake, 1993). Thus, for example, one is able to estimate accurately when it is safe to cross a street in the presence of oncoming traffic without being able to verbalize the quantitative values of the width of the road, the distance to the approaching vehicle, or the speed of locomotion (Lee, 1976). Similarly, while driving, the driver may be capable of successfully accomplishing the nonverbal task (maintaining a safe headway) but not the verbal task (i. …

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