Intelligent speed control and effects on driving behaviour

Supporting the driver in conducting his nowadays demanding task is a promising means to get the maximum out of the road system with respect to both efficiency and safety. With respect to safety, speed management is a main issue. Police enforcement of speeding is one approach, preventing high speeds another. Intelligent Cruise Control (ICC) Systems may enable this by operating at an externally induced reference speed limit. ICC is an in-vehicle system that automatically regulates a vehicle's speed and is capable of maintaining a proper following distance behind a lead vehicle. ICCs so far can realise a moderate level of deceleration: in situations that require higher the driver must take over control. Some kind of short-range communication with the road side offers the option to obtain in-car preview information about relevant conditions on the road ahead, including prevailing speed limits). Some driving simulator studies were conducted to assess the effects of ICC on driving behaviour for both informative and intervening ICC systems. The results reveal that ICC yields more consistent longitudinal control: in car-following situations, there is less variation in headway and in speed. Only intervening systems result in a speed reduction on motorway sections with a special speed limit, but at a cost of somewhat higher speeds at non-controlled sections. In critical scenarios (approaching a sudden traffic queue) where ICC could not cope with completely and the driver had to take over control, a somewhat later braking reaction of the driver was found. Also within the European project MASTER (MAnaging Speeds of Traffic on European Roads) the testing of the testing of (dynamic) speed limiters in a simulator study resulted in a reduced speed, speed variance and speed at hazardous locations. However, the speed limiters also had secondary effects that could compromise safety, viz. a higher incidence of short headways, delayed braking and a higher incidence of collisions. A field-trial with an in-vehicle dynamic speed limiter in three countries (Sweden, The Netherlands, and Spain) revealed that the speed limiter significantly reduced speed on 30-70 km/h roads, but not on 80-90 km/h roads and motorways (mainly due to heavy traffic conditions). In conclusion, automatic speed limiting by an in-car device seems most promising within built-up areas. Some concern about behavioural adaptation mechanisms may apply.