Speed photo enforcement effects on headways in work zones

Determining the effects of Speed Photo-radar Enforcement (SPE) on headways of vehicles traveling in a work zone is important since maintaining a larger headway provides drivers with a longer time to avoid a potential rear-end collision. In addition, when the headway between two vehicles is less than the follower's reaction time, a rear end collision is likely to happen if the leader suddenly decreases its speed. In this paper, the effects of the SPE on the headway of vehicles traveling along a work zone are studied. For this purpose, we collected data from two locations that were around 1.5 miles apart in a work zone on interstate highway 55 at. The SPE van was present at the upstream location. The headway of all vehicles within a period of 45 minutes was measured at the upstream location as well as the downstream location. The results of this study showed that the presence of the SPE did not reduce the headway of the traffic stream at the location that it was present, as well as at the downstream location. This was confirmed by comparing the headway of followers (headway less than four seconds) when the SPE was deployed in the work zone to the headway of them when the SPE was not present, for both lanes combined, and for the shoulder and median lanes separately. The same result was observed for different pairs of leaders and followers. In addition, our analysis showed that the SPE prevented a reduction in the headway of car-car pairs traveling on the median lane at the downstream location.