Driver headway: How close is too close on a motorway?

Driver headway has recently become an important question with much attention being given to unsafe headways or 'tailgating'. This paper reviews a series of recent studies undertaken at the University of Southampton, which have sought to measure and model distance keeping, demonstrating how following distance depends on a wide range of factors, some of which are only recently being explored. These include variations in following distance for any particular driver and the relationship with time to collision, variations in following distances of drivers of differing nationalities and the ability of the driver to 'read the road ahead', which may be affected by interaction with different vehicle types. It is demonstrated that providing clear unequivocal statements regarding car following and safety levels, even after such studies, is still far from straightforward.