Link travel times 1: Desirable properties

When modelling travel times and traffic flows varying over time on traffic networks it is desirable that the travel times should satisfy certain properties. This paper investigates these properties and their interrelationships, in particular, uniqueness and continuity, first-in-first-out (FIFO), causality and time-flow consistency, while assuming a single traffic type. For generality we investigate these properties independently, as far as possible, of how the travel times have been defined or derived. In a companion paper (Carey, 2003) we investigate the same properties for a particular class of travel-time model, namely travel times derived from exit-flow models.