The effects of changes in the traffic scene during overtaking.

Overtaking maneuvers were studied in real traffic, by comparing cases where a change in the opposite traffic occurred during the overtaking maneuver i.e., appearance of an oncoming car, with cases where no change occurred during the maneuver i.e., either an already apparent oncoming car or no oncoming car. In total 45 naturally occurring cases of overtaking were analysed. By examining the time headways (TH) between the overtaking car and the other cars involved, at the end of the maneuver, a significant correlation was found between the TH to opposite traffic and the TH rear to the overtaken car. In cases where a change in the opposite traffic occurred and the proximity to opposite traffic was below a critical TH (<4s) the appearance of a new-oncoming car caused the overtakers to actively press, both longitudinally and laterally, close to the overtaken car. In cases of overtaking where no change in the opposite traffic occurred, the TH rear to the overtaking vehicle was unaffected by the TH to the opposite traffic. Interestingly, in comparing cases where an oncoming car appeared during overtaking with cases where an oncoming car was apparent from the start of the maneuver both the TH to the opposite traffic and the duration of the maneuver were not found to be significantly different. It is suggested that drivers were probably expecting to be confronted with an oncoming car during the overtaking. However, the decreased available time to disambiguate this situation leads the overtaking driver to limit the rear safety margin of the vehicle being overtaken. The appropriateness of this practice, in terms of safety, remains questionable.