Headway estimation

Taieb-Maimon, M. (2007). Learning headway estimation in driving. Hum. Fact, 2007, in press. 

Resist films 0.5 to 1 ym thick were spin-coated on 3-inch diameter silicon wafers (oxide coated). The polymers were applied from 10% solutions in chlorobenzene, 2-ethoxyethanol, or 2-methoxyethyl acetate using a Headway Research Model EC-101D spinner. Prebaked (1 hr, 80°C, vac.) films were exposed to the e-beam and then extracted with about 4 ml of tetrahydrofuran, THF. About 4 to 8 hrs was allowed for dissolving to take place. Molecular weights were estimated using a Waters Model 201 HPLC equipped with 4 y-Styragel columns of nominal sizes 500, 10, ... 

Road users have the tendency to nnderestimate the risk posed by fog, snow and rain, by not snfficiently adjnsting their behaviour (e.g., by decreasing speeds, and adopting larger headways) to the environmental conditions. This is clearly shown in a Finnish snrvey stndy. On actnal slippery road surfaces, only 14% of the drivers estimated the road to be slippery, while more than half considered the friction to be normal (Heinijoki, 1994). Average speeds on slippery roads are not sufQdently low to compensate for the effect of weather (Edwards, 1999), or for the reduced friction (Malmivuo and Peltola, 1997 Roine, 1993 Saastamoinen, 1993 Varhelyi, 1996). Also, headways are not snbstantially affected by winter conditions (Saastamoinen, 1993). Therefore, roadway ITS have been introduced to overcome this tendency of drivers to underestimate the risk in the case of adverse weather conditions. 

If parameters such as average headway and average speed, averaged across large area, are used to support the modelhng, the outputs will be of sufficient qrrality to permit coarse estimation of risk flnctiration caused by replacement of track circuits by axle counters. 

In a series of studies that we conducted in Israel we looked at drivers choices of safe and comfortable headways, tiieir ability to verbally and non-verbally estimate headways, the relationship between the headways drivers keep and their skills, their ability to improve their judgments, and the potential for feedback devices as learning tools to increase headways. The following is a brief description of these studies and their results. 

Drivers estimation of minimum safe headways and comfortable headways... 

In the first study (Taieb-Maimon and Shinar, 2001), experienced drivers with Snellen visual acuity of 6/9 or better were asked to drive on a four lane divided highway behind a lead vehicle. An experimenter that drove the lead vehicle adjusted its speed in a random fashion from 50 to 100 km/hr. At each speed, the driver in the following car was asked to follow the lead car by keeping a minimal safe distance at vdiich he or she would still be able to stop in time should the driver of the lead car bre suddenly . Once the drivers reached that headway they were asked to estimate that gap - either in terms of meters, car-lengths, or seconds. Then the drivers were asked to slow down so that the gap widened significantly. They were then asked to follow the lead vehicle at what they considered a comfortable distance. Once this procedure was completed the lead driver selected another speed and the whole sequence was repeated. 

If drivers are able to adjust their headways in order to keep the same safety margin at all speeds, why do they keep them so short One possibility is that they underestimate the actual headway. Some support for that was found when we analyzed their verbal estimates of their headways. All drivers invariably overestimated their headways by an average of 0.24 seconds... 

One method to improve headway judgments would be to have in-vehicle headway-o-meters (just as we have speed-o-meters). The belief that minimum headways can be regulated and enforced rests on the assumption that drivers are capable of either directly perceiving or correctly estimating their headways. We do not make that assumption with respect to speed and that is why we have speedometers in our cars. The research on drivers headway judgments shows that we are incapable of this task too, and need some kind of an aid. 

With these results in mind we can now state, with a significant amount of confidence, that (1) in the absence of feedback, drivers tend to keep headways that are significantly shorter than the recommended safe headways, and often shorter than their brake reaction time, (2) they overestimate their headways, so that they may actually believe that they are safer than they are, (3) with objective feed back, drivers are able to learn to estimate their time headways fairly accurately, and (4) once that learning occurs, it can be retained for long periods, at least as long as 6 months. 

As often done in behavioral research, we also analyzed the effects of gender and age. In all three studies (Taieb-Maimon and Shinar, Ben-Yaacov et aL, and Shinar and Schechtman) there were no consistent significant differences between males and females and no differences between the younger and older drivers. This indicates, that men and women do not differ in the perceptual-cognitive skills that are needed to estimate headways, and that age or driving experience is not a relevant issue - at least as long as all drivers are neither young novice drivers nor older than 60 years old. 

The Power and exponential models are used here for linking the accident risk to any single variable - speed, occupancy, time headway. We call POWER Ra a Power model where a second explanatory variable - the rain occurrence - is introduced the parameters are estimated on its logit form, where this model reads ... 

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