Models of travel behaviour are necessarily based on assumptions about human decision making behaviour, particularly decision rules and knowledge states of decision makers. The travel time between two points in a network will almost certainly depend on the route choice. The likelihood of consequences or the uncertainty of travel alternatives depends on the knowledge state of the decision makers and the external processes which generate this uncertainty. Within transport modelling it is generally assumed that travel time is one of the major criteria drivers use to choose a route. Empirical studies on this subject do not always confirm this assumption at first sight. Travel time will probably be one of the most important criteria drivers use to choose a route. Furthermore, it seems possible to use variance to represent uncertainty in travel time.