Public transit planners rely on measurements of network performance to anticipate the impact of changes to a transportation system. Accessibility-based measurements emphasize how well a transportation system allows individuals to reach desired opportunities, rather than maximizing network properties such as capacity. This paper presents a measurement of accessibility for transit customers making unanticipated, spontaneous trips. Measuring this spontaneous accessibility was facilitated by developing an open-source software tool that can analyze a transit network throughout an entire day, over a complete municipal boundary or transit agency service area, at fine spatial granularity, and without some of the simplifying assumptions made by previous studies. The tool is used to study spontaneous accessibility within the city of Seattle over a one-year period featuring the opening of a light rail extension and restructure of bus services. Studies of this nature require only limited data sources but produce precise results, and thus can be utilized to measure iterative refinement of the transit network. Furthermore, techniques from the discipline of information theory provide insight into ways to reduce the computational demands, giving planners the ability to consider more alternatives.
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