A system and method of assessing the driving task demand on the driver of a vehicle, and further controlling one or more devices on the vehicle as a function of the assessed driver demand is provided. The method includes sensing a coverage zone in relation to a vehicle, determining a presence of one or more objects in the sensed zone, measuring speed of each detected object in the sensed zone, determining a variation in speed of one or more sensed objects, and determining a driving task demand signal indicative of driving task demand of the vehicle as a function of the measured speed variability. The method controls one or more devices on the vehicle based on the driving task demand signal. Alternately, the driving task demand signal is determined based on vehicle speed.