Mapping transit-based access: integrating GIS, routes and schedules

Accessibility is a concept that is not entirely easy to define. Gould (1969) once stated that it is a 'slippery notion … one of those common terms that everyone uses until faced with the problem of defining and measuring it'. Considerable research over the last 40 years has been devoted to defining and measuring accessibility, ranging from access to jobs within an hour's travel time to the ease at which given places can be reached. This article is concerned with the measurement of access provided by transit. It includes a review of past work on measuring accessibility in general and with respect to transit services in particular. From this overview of the literature, it can be seen that current methods fall short in measuring transit service access in several meaningful aspects. Based on this review and critique, we propose new refinements that can be used to help overcome some of these shortcomings. As a part of this, we define an extended GIS data structure to handle temporal elements of transit service. To demonstrate the value of these new measures, examples are presented with respect to mapping accessibility of transit services in Santa Barbara, California. Finally, we show how these measures can be used to develop a framework for supporting transit service analysis and planning.