More than travel time: New accessibility index capturing the connectivity of transit services

Various qualities of transit services, including travel time, frequency, and the presence of alternative paths, affect accessibility—how easily passengers can reach their destinations by transit. Traditional accessibility metrics, such as gravity-type and cumulative-opportunity indices, rely exclusively on travel time as a proxy for travel impedance, failing to consider other qualities of transit services. This study develops a comprehensive transit-based job accessibility index that accounts for the number of feasible alternative routes and their frequency in addition to in-vehicle and out-of-vehicle travel times. It uses the General Transit Feed Specification (GTFS)data and Dijkstra's shortest path algorithm to compute these components of tract-to-tract transit trips in the Chicago Metropolitan Statistical Area (MSA). We demonstrate that conventional accessibility indices considering only gross travel time overestimate job accessibility in the job rich downtown area and suburban train hubs, while they tend to underestimate job accessibility in places mainly served by a suburban bus system. Logistic regression analysis shows that our refined job accessibility measure explains the likelihood of transit commute considerably better than conventional measures.