Ichikawa, K., Kagawa University, Japan
The purpose of this study was to measure the time headway between vehicles to get information regarding safe distance while driving. The safe distance taught in Japanese driving schools is based on the concept of "stopping distance", which varies according to the speed of the cars. However, we usually cannot accurately perceive distances in meters while driving. In the UK, USA, Australia and New Zealand, the time gap (time headway) is taught as a cue to driving safely. We observed 2,004 vehicles running on suburban and city roads in January 2001. The roads we observed had two lanes, this being a cruising lane and an overtaking lane. As to weather variable, we selected two weathers to observe during raining and no rain (cloudy) in both conditions. We video-recorded the vehicles to analyze for 20 minutes in each of the locations both during the day and at night. We operationally defined the criterion of "fleet" when a group of fleet ran holding more than 5 seconds time headway in front and backward interval. An ANOVA attained three main effects, which indicated significantly shorter time headway of vehicles running in cloudy than in raining, at suburban than city road, and at overtaking than cruising lane. The safe distance at 60 km/h taught at driving schools in Japan was tentatively equivalent with time interval of 2.70 seconds. Nearly 80 % of overall vehicles maintained unsafe time headway.