Substitutability as a spatial concept to evaluate travel alternatives

In this paper we propose the concept of ‘substitutability’, which we define as the extent to which the preferred travel alternative can be substituted by other initially less preferred alternatives. This is particularly of interest when the preferred alternative is no longer available, e.g. due to labour strikes, weather conditions, power failures, etc. Travel alternatives in this context can comprise of activities, modes, time of day, and routes. We argue that substitutability is a promising new concept, which is relevant for travel behaviour research. In particular, substitutability is relevant from an accessibility perspective, as well as from the perspective of ‘freedom of choice’. In this paper we conceptualise the concept of substitutability, present a mathematical expression for it and discuss its relationships with other related concepts in the travel behaviour research field, such as the freedom of choice, accessibility, and robustness/reliability. We illustrate the concept of substitutability using a case study, where we look at the extent to which airports can be substituted by other airports, and by high-speed railway stations, conditional on a given destination, namely the cities of Paris, London and Frankfurt. Finally, we present a research agenda.