Towards accessibility thresholds

A major issue with the use of accessibility measures in transport studies is that it is not clear how to define the level of accessibility that people should have, and below which they may not be able to function within society. Although there are myriad studies on the impacts of low or unequal accessibility, only a few have made efforts to specify which opportunities must be reachable, how much choice is required, or under what circumstances access should be protected. As a result, the boundary below which accessibility becomes socially inadequate is often left implicit. This paper provides the first theoretical framework for defining accessibility thresholds grounded in Sen’s Capability Approach, theories of freedom, and existing conceptual work on sufficiency and thresholds. We identify the capabilities that accessibility must support, classify the conversion factors that condition their realisation, and propose normative principles—informed by philosophical accounts of freedom—to guide where and how thresholds should be set. We then outline a practical sequence of steps enabling researchers and practitioners to adapt existing accessibility measures to reflect what people have reason to value. By making explicit the ethical commitments underlying accessibility thresholds, this framework supports more transparent, just, and context-sensitive evaluation of transport systems.