Isolating the Role of the Transport System in Individual Accessibility Differences: A Space-Time Transport Performance Measure

Abstract

Accessibility differences across individuals are a core topic in the transport equity debate. Space-Time Accessibility measures (STAs) have often been used to show such differences, given their sensitiveness to individual spatial and temporal constraints. However, given their complexity, STAs cannot properly isolate the specific role of the transport system in individual accessibility differences, since it is mixed with several other spatial, individual and temporal factors. To isolate the role of the transport system, this study introduces a Space-Time Transport Performance measure (STTP) that (a) grounds on the individual daily schedule of fixed activities, (b) calculates the generalised transport costs each individual has to bear to perform such schedule, and (c) weights it against the Euclidean distance between the activities of such a schedule. STTP is tested together with STA for a small sample of individuals living and performing their daily activities within the 22nd district of Vienna. This test provides two main findings: first, individual differences registered by STTP tend to be smaller than those highlighted by STA, according to the former’s more narrowed and transport-specific approach. Second, individuals with the highest STA do not necessarily register the highest STTP (and vice versa). Indeed, some may experience limited transport performances when running their mandatory daily schedule, while registering a high degree of access to discretionary activities according to their constraints and opportunities at disposal (and vice versa). Considering these results, STTP may be seen as a complementary indicator to be used together with STA to analyse both general and transport-specific individual accessibility differences. Its role is particularly important for transport policy makers, who should understand which accessibility differences are directly linked to the performances of the transport system and could be remediated through transport policies.