Mental load and loss of control over speed in real driving.: Towards a theory of attentional speed control

Relationships between attention and speed control are analysed. The demands of speed control are derived from subjective appreciation of the traffic environment and from the need to adapt to legal restrictions. Drivers check the speedometer more or less frequently and also use their subjective speed perception while looking ahead. It is hypothesised that, in absence of specific speed restrictions, drivers choose an optimum preferred speed so that the attentional effort dedicated to speed control is minimised but with explicit speed restrictions additional resources are required to keep speed under control including more glances to the speedometer. If a secondary task needs attention the speed control is affected and the speed tends to return to the optimal preferred level. Twelve participants drove about 200 km in a Spanish highway under eight conditions: two speedometer states (visible/concealed) × two speed instructions (free/restricted between 90 and 100 km/h) × two workload states (normal driving/performing a mental task). The results demonstrated: (a) that under free speed the drivers choice was 11 km/h faster than in the restricted-speed condition, indicating that the speed instructions were effective and operated as a speed limit. (b) When a mental task was performed the speed increased independently of the speedometer availability under restricted speed but not in the free speed condition, what clearly supports the stated hypothesis. The role of the speedometer in speed control, its relations with endogenous distraction and its implications for driving are analysed.