Academic journal article Human Factors

Headway Feedback Improves Intervehicular Distance: A Field Study

Academic journal article Human Factors

Headway Feedback Improves Intervehicular Distance: A Field Study

Article excerpt

INTRODUCTION

The ability of people to remain alert in a continuous manner for prolonged durations is severely limited, and consequently momentary lapses in attention and delayed recognition of hazards have been shown to be a primary contributor to a variety of crashes. According to early estimates (Sabey & Staughton, 1975; Treat et al., 1977), inattention and related factors such as improper lookout and distraction are responsible for approximately 50% of police-reported crashes. According to a more recent estimate, based on analysis of a sample of 723 crashes in the United States investigated by the National Automotive Sampling System, inattention, improper lookout, and failure to keep a safe distance constituted 23% of all unsafe driver actions that were responsible for these crashes (Hendricks, Freedman, Zador, & Fell, 2001).

Rear-end crashes typically constitute approximately 30% of all police-reported crashes (National Highway Traffic Safety Administration [NHTSA], 1999), and approximately 90% of them can be attributed to inattention, following too closely, or both (Knipling, Mironer, et al., 1993; Knipling, Wang, & Yin, 1993). Even an alert driver may fail to maintain a safe headway from the car ahead because of a perceptual inadequacy in estimating headways (Taieb & Shinar, 2001). Maintaining a safe headway while driving is considered a key determinant of defensive driving. A standard component of most licensing manuals is concerned with educating drivers about perception-reaction time, vehicle stopping distance, and the implication of the two for safe headways from vehicles ahead. This headway is typically defined in terms of time rather than distance, and a commonly recommended minimum safe headway is 2 s. That way, if a lead driver initiates a braking action, the following driver has 2 s to initiate a braking response to t he slowing down of the vehicle ahead.

In contrast to the licensing manuals' recommendations, past observational and experimental research has shown that normative behavior on the road often violates this standard. Observational studies of unaware drivers in the United States have shown that headways of 1.0 s or less may be much closer to the norm than headways of 2.0 s or more (Chen, 1996; Evans & Wasielewski, 1983). According to Evans (1991) there are two possible reasons for this. First, drivers keep such small headways because as long as a lead and a following vehicle maintain the same speed there is no risk of collision. Second, most drivers' past experience has reinforced them for driving with short headways. Thus both explanations are based on the assumption that drivers maintain an acceptable level of risk in their driving behavior.

There is, however, one other reason for the short headways, and it is that drivers are incapable of estimating headways correctly. Two recent experimental studies conducted on public roads suggest that this is in fact the case. In the first study (Taieb & Shinar, 2001), drivers were instructed to drive as close as possible to a lead vehicle but still be able to stop if necessary. Once they reached this headway, they were asked to estimate it in terms of seconds, car lengths, or meters. Under such instructions, the mean actual time headway THW was only 0.66 s. In contrast, the mean estimated THW (when estimated in seconds) increased from 1.9 s when the lead vehicle traveled at 50 km/h to 2.6 s when the lead vehicle traveled at 100 km/h. When asked to drive at a "comfortable" headway, these drivers increased their mean THW to 1.1 s, a headway that is more consistent with that observed in real traffic than is the headway recommended by safety manuals. In the second study (Ben-Yaacov, Maltz, & Shinar, 2002) driv ers were instructed to maintain a following distance of 1.0 s whenever they approached another vehicle traveling ahead of them. Again, most drivers approached to significantly shorter headways. …

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