Application: Accessibility as a System Performance Metric

Performance metrics currently in use by DOTs, such as volumes, hours of delay, reliability indices, and so on, concentrate on mobility. That is, they measure the performance of the network in terms of how vehicles are traveling on it. There is a twofold problem with these measurements. First, they treat faster vehicle movement as the priority, which runs counter to other goals of DOTs. Safety is reduced at higher speeds, other things being equal. And infrastructure which facilitates moving vehicles quickly is a barrier to moving at all in more sustainable modes (walking, cycling, and public transit). Second, even with respect to auto trip making, mobility metrics fail to distinguish the importance of delay and congestion in the context of origins and destinations. As discussed above, if congestion increases but does not materially impact the destinations reachable in a reasonable time, is there really a performance issue?

Accessibility metrics consider both origins and destinations. Importantly, the access to destinations measured for each origin reflects potential connection, not observed trip-making. This means the accessibility metrics are measuring the combined impact of land use and transportation infrastructure, on what opportunities are being provided to residents and constituents. It also provides context to congestion, which does after all reduce accessibility. Measuring accessibility directly means an evaluation of the transportation and land use systems as a whole, not a limited analysis of roadway segments of high demand.

Accessibility metrics such as the number of job destinations reachable in 30 minutes, have useful qualities for performance evaluation: they are comparable and consistent, people-scaled, and can measure outcomes.