Effects of ACC on driver behaviour, workload and acceptance in relation to minimum time headway.

Effects of Adaptive Cruise Control (ACC) on driving behavior, workload and acceptance were studied in inter-urban environments. Interactions with ACC minimum time headway were also analyzed. A driving simulator study with 24 participants, aged 23 - 55 years, was conducted. A mixed design was used - all participants performed test drives both with and without ACC, whereas each participant was exposed to only one of three ACC minimum time headways, 0.8s, 1.0s, and 1.5s. The total driving distance was 120 km, divided into two rural road sections and two motorway sections. Along the route, "normal" as well as more "safety critical" scenarios were presented. Some effects of ACC on driving behavior were found: the maximum driving speed was reduced. Speed variability was reduced when there was no other traffic around. On the motorway the distance driven in the left driving lane was increased. Minimum time to collision was reduced for two non-critical scenarios on the rural road and for one non-critical scenario on the motorway. As to workload, the participants rated lower physical demand and effort when driving with ACC on the motorway. Regarding user acceptance, the participants reported the ACC to be quite useful and quite pleasant to use. Most participants reported that they were willing to pay between 100 -500 Euros for having the ACC installed in their car. The participants also participated in a test intended to study their preferred ACC minimum time headway. Their average preferred headway was between 1.5 and 2.5s on the motorway and between 1.7 and 3.2s on the rural road. A supplementary study was performed with eight participants, aged 24-42 years, who drove with and without ACC support in two motorway sessions. One ACC minimum time headway was used, 1s. The same effect measures as in the major study were used. Besides, heart rate measures (Inter Beat Interval, 0.1Hz component of Inter Beat Interval variability) and performance on the Peripheral Detection Task (PDT - a secondary task performance measure) were measured as indicators of workload. For three scenarios heart rate decreased as an effect of ACC support.