Reuniting spatial interaction and accessibility
For a long time I felt there was a missing link between spatial accessibility and spatial interaction models. We finally have a paper that helps us understand the common historical roots of these two streams of literature and why they diverged. Our new study (preprint) goes down memory lane to show what spatial accessibility research can learn by looking into the past.
The paper shows that the proportionality constant in spatial interaction models
was dropped in the early days of spatial accessibility research. This omission
created fundamental problems that undermine the interpretation, communication,
and comparability of accessibility analysis, particularly gravity-based metrics
thanks Hansen.
The study shows that bringing the proportionality constant back into accessibility allows us to think of a new “family of accessibility measures” derived from spatial interaction principles. In practical terms, this fixes those fundamental problems, making sophisticated gravity-based and competitive accessibility more interpretable, widely adoptable, and policy-relevant
The paper was written with a stellar team led by the incredible Anastasia Soukhov.
ps. It’s been a while since I’ve learned so much from a collaboration.
- Soukhov, A., Pereira, R. H. M., Higgins, C. D., & Paez, A. (2025). A family of accessibility measures derived from spatial interaction principles. OSF Preprints https://doi.org/10.31219/osf.io/a9dxb_v1
Abstract:
Transportation planning has long prioritized the efficiency of movement, or mobility. However, the concept of accessibility represents a more comprehensive evolution, shifting focus from mere movement to the potential to reach (i.e., spatially interact) with desired destinations. Despite growing recognition of accessibility-based planning approaches, the concept remains fragmented, with inconsistent definitions and unclear interpretations. This work’s aim is to clarify and unify the concept of accessibility by connecting it into spatial interaction modeling. We demonstrate that widely used mobility and accessibility models, such as gravity-based accessibility and spatial interaction models, share common theoretical roots. From this foundation, this paper offers three contributions: (A) we introduce a family of accessibility measures within the principles of spatial interaction, and (B) formally define four members of the family, namely the ‘unconstrained’ measure (i.e., Hansen-type accessibility), the ‘total constrained’ measure (i.e., a constrained version of the Hansen-type accessibility), the ‘singly constrained’ measure (i.e., related to the popular two step floating catchment approach - 2SFCA), and the ‘doubly constrained’ measure representing realized interactions or ‘access’, effectively equal to the doubly constrained spatial interaction model; and (C) we demonstrate the interpretability advantages of the family, as these constrained accessibility measures yield values in units of the number of potential “opportunities for spatial interaction” or “population for spatial interaction” for each zone and zonal flow. The family of accessibility measures proposed here clarifies the concept of ‘potential’ in accessibility, demonstrates theoretical and formulaic linkages across popular accessibility and spatial interaction models, and reintroduces measurement units into accessibility measures. By doing so, we believe this family of measures can unlock a clearer, more interpretable, and cohesive foundation for accessibility analysis.
