The relationship of safety and headways in three Helsinki suburban streets was studied. Moreover a method for automatic measuring of headways was tested. The mean headway on one of the streets with an average traffic flow of 800 vehicles per hour was about four seconds. On two other streets with 200 vehicles per hour the mean headway was 16 seconds. Very short headways (less than 1 second) were not correlated with an hourly traffic flow. Rear-end collisions on the street with 800 vehicles per hour were the second most frequent accident type, where as no rear-end collisions were reported to the police on the other roads during 1979-1984. An analysis of conflicts showed the risk of rear-end collisions to be highest on the road with 200 vehicles per hour. On the street with 800 vehicles per hour, however, all conflicts registered were of the rear-end type. Photocell detectors connected to a data logger proved to be, with some reservations, suitable for measuring headways in urban traffic conditions.
Abstract