Safety margins in the driver

Sammanfattning: The primary aim of this thesis is to highlight the most important features of driving and to describe the models that have attempted to conceptualise these features. The discussion focuses on the concept of "safety margin." The concept is elaborated upon in an effort to enhance its usefulness as an empirical tool in traffic research. In this study, safety margin is defined as a threshold value that informs the driver when to undertake an action to minimise the risk of a car accident. Three separate studies of various driver behaviours are presented as illustrations of how this view can be applied in a real highway traffic setting. One study (Study I), consisting of three independent but related experiments, examines car following; a second study (Study II) explores gap acceptance at a T-crossing; and a third study (Study III) investigates drivers’ braking decisions.The overall findings of the present studies suggest that it is valid to model driver behaviour as a concern related to the control of safety margins. It was shown that the driver controls time-distance dynamics to leading and following cars when driving in a queue. A bias in the drivers’ impressions of distances to leading and following vehicles that has safety promoting implications was also found. There was no evidence for the hypothesised use of the limit for dissolution of time-distance to oncoming vehicles for merging decisions at a T-junction. Drivers’ rules for establishing braking decisions were successfully assessed in a field study using linear regression.