Application: Accessibility Measures — Outcomes of DOT Action

Transportation agencies should seek to measure their performance in terms of what is provided to residents as a function of their decisions. Accessibility metrics fulfill that requirement by quantifying the outcome of decisions in terms of travel opportunities, across different modes, destination types, geographic locations, time periods, and population characteristics. 

System performance metrics should reflect improvement across all areas of DOT responsibility. Relative investment in different modes will produce different opportunities for people using those modes, which is captured fully by comparing accessibility at origins across modes. Roadway infrastructure improvements can create additional accessibility, as can improvements to bicycle facilities, walking paths, and transit frequency. Changes in access can be compared at the same scale, or on a comparative (percent change) basis over time or across geographic areas. The relative return of accessibility from DOT investments becomes straightforward, as units of bike accessibility match auto accessibility, just as easily as the dollar value of freeway investments can be compared to dollars spent on separated bikeways.

As acknowledged in Figure 1.3, accessibility is influenced by (and influences) both land use and travel behavior. However, the strong, direct connection between infrastructure and accessibility reinforces the idea that the accessibility is a good outcome metric for infrastructure. Travel behavior changes are downstream of changes in accessibility, and congestion and delay metrics are downstream of those individualized decisions. In comparison to the diffuse, lagged, and mediated impact of infrastructure on metrics like segment speed variability, accessibility changes definitionally with changes to the transportation network. The final advantage of this approach is that unlike the travel behavioral measurements of vehicle congestion, accessibility can be measured on planned investments before they happen, providing a baseline expectation to compare against once projects are completed and opened, and before the induced demand impacts of land use change and travel behavior appear.