Substitutability as a concept to understand travel behaviour and its implications

In this paper we propose the concept of Substitutability and define it as ‘the extent to which one or multiple characteristics of the preferred travel behaviour alternative can be substituted by alternatives’. Characteristics include activity choice, mode choice, time choice, and route choice We argue it is a promising concept, which is relevant for travel behaviour research. In addition, it is relevant from an accessibility perspective, and from the perspective of ‘freedom of choice’. We conceptualize the concept, present a mathematical expression, discuss its relationships with the concepts of the freedom of choice, accessibility, and robustness/reliability. We argue substitutability is a gradual concept, that can be measures pre trip, on trip, and while executing an activity pattern. It can be analysed at different levels: (1) components of trips for one person, (2) a full trip or activity for one person, (3) a cluster of activities/ trips for one person, (4) an aggregation of the three levels above, but now for a group of persons, and (5) the perspective of the origin of the trip or the perspective of the destination. In addition, we discuss modelling substitutability and its use for evaluation purposes, and we present a research agenda.