A general model of diverging from a freeway to an exiting area was developed on the basis of purely behavioural considerations. The model proposes three sequential response elements that together define speed-change lane length. The first element is a criterion for a driver to diverge from the freeway on the basis of the angular velocity of the exit ramp gore (relative to the driver while approaching the exit in the right lane of the freeway). The second element is the distance required for drivers to complete a steering-control manoeuvre onto the speed-change lane. The third element is the distance at which drivers begin to brake in order to move from tracking the speed-change lane to tracking the curve of the exit ramp. A mathematical definition of each element was developed. These definitions allowed the prediction of each of the critical distances. The predicted distances were compared to observed exiting behaviour on both curved and tangent exit ramps. The diverge, steering-control, and begin-braking distances observed on curved ramps were not statistically different from those predicted by the model. The begin-braking distance estimate for tangent exit ramps was not validated, probably because of instrumental errors in the field data analysis. The results indicate that the model is a reasonable representation of the exiting manoeuvre and provides a rational means for the design of exit speed-change lanes in terms of its ability to predict how far upstream from the wedge point the speed-change lane should begin, how far from the wedge point the steering-control manoeuvre ends, and how far from the wedge point the driver will initiate braking for a curved ramp. This paper appears in Transportation Research Record No. 1281, Human Factors and Safety Research Related to Highway Design and Operation 1990.
Abstract