Reuniting Accessibility Measures with Spatial Interaction Principles

Transportation systems play a fundamental role in facilitating access between people and opportunities across place. Place-based accessibility can be defined as the potential of opportunities for spatial interaction. For decades, transportation planning research and practice have relied on mobility-based metrics, such as kilometers traveled, emissions released, or vehicle counts, measures that reflect realised movement. In contrast, accessibility focuses on potential movement, such as the number of opportunities that can be reached within given constraints. Although there has been growing interest in shifting from mobility-based methods to access-focused planning approaches, a few barriers remain. One significant issue is methodological, namely, the lack of clarity in how accessibility values are interpreted. There are several approaches to match values with meaning, but this dissertation proposes a preceding step: clarifying the units of accessibility by introducing proportionality constants derived from spatial interaction principles. In response, this monographic-style dissertation is organized into six chapters. In the first chapter, a review of how accessibility literature largely diverged from the spatial interaction literature is detailed. This lays the foundation for how the addition of proportionality constants that return the units to the measure and balance them to reflect known constraints in the system may be useful. In the second chapter, using a synthetic example, the total constrained accessibility measure is introduced. This measure is formally proportional to unconstrained accessibility (i.e., Hansen-type accessibility measure commonly used in literature) but with results consistently expressed in units of accessible opportunities or units of accessible population. Then, using the same synthetic example, the single constrained accessibility measure is introduced. This measure considers competition for opportunities and could be understood as related to the popular measures of competitive accessibility. Multimodal extensions as well as the expression of these measure in terms of ‘market potential'--the accessible population at a zone--is also introduced. In the third chapter, motivation, data and methods of an empirical example of the parkland area and population in the City of Toronto are detailed. All members of the family of accessibility measures introduced in this work are compared and contrasted in the subsequent chapters using this empirical example. In the fourth chapter, a unimodal (walking only) case considering a binary impedance function and all population is presented, where the accessible parkland area and accessible population in Toronto is quantified. Differences in the unconstrained (i.e., conventional method) and constrained measures are detailed. Fifth, a multimodal case of accessible parkland area and accessible population is detailed, considering walking, cycling, transit, and car modes. Similarly, differences in the unconstrained and constrained measures are detailed. In the sixth and final chapter, a summary of key points across all chapters is provided along with a thorough discussion on how constrained accessibility improves communicability and details on future lines of potential investigation.

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