Index of braking behaviour in two dimensions within risk perception

Time To Collision (TTC) and other physical indices are widely used to predict drivers' braking behaviour. However, these indices often overlook the psychological aspects of how drivers perceive risk. Moreover, such indices are predominantly one-dimensional and do not account for the shared space, where vehicles can move in two dimensions. In this study, we introduce an index designed to predict drivers' braking behaviour in the shared space, taking into account the drivers' risk perception. In Experiment 1, a functional equation was derived to estimate the probability that drivers would brake in a given situation. The results confirmed that the index provides an adequate reflection of drivers' risk perception. In Experiment 2, the braking rate estimated with the index provided a better fit to a third -person risk evaluation when compared to other physical indices.