Measuring accessibility: positive and normative implementations of various accessibility indicators

Accessibility is a concept of continuing relevance in transportation research. A number of different measures of accessibility, defined as the potential to reach spatially dispersed opportunities, have been proposed in the literature, and used to address various substantive planning and policy questions. Our objective in this paper is to conduct a review of various commonly used measures of accessibility, with a particular view to clarifying their normative (i.e. prescriptive), as well as positive (i.e. descriptive) aspects. This is a distinction that has seldom been made in the literature and that helps to better understand the meaning of alternative ways to implement the concept of accessibility. Our discussion of the positive and normative aspects of accessibility measurements is illustrated using the city of Montreal, Canada, as a case study. The example highlights the differences in the measured levels of accessibility depending on implementation. Comparison of the two by means of a relative indicator of accessibility helps to identify the gap between desired (as normative defined) and actual (as revealed) accessibility levels.