Testing Site Metrics: Spatial Accessibility


Spatial accessibility to COVID-19 testing is an indicator of whether residents of a particular neighborhood have access to nearby COVID-19 testing. We used data provided by Castlight Health Inc. containing the location of testing sites in the 30 BCHC cities to construct two spatial accessibility measures:

  1. A walkshed-based measure: a neighborhood is defined as having high access if at least 25% of its area is within a 15 minute walk of a testing site.
  2. A 100-meter buffer based measure: a neighborhood is defined as having high access if any portion of it is within 100 meters of the site.

For this blog post, we will use Philadelphia as an example to demonstrate how these two definitions of spatial accessibility were implemented and to illustrate the results. The locations of testing sites for Philadelphia as of January 2021 are displayed on the map on the left as blue dots. We defined neighborhoods using the U.S. Census Bureau’s block groups; the block group boundaries (gray lines) for Philadelphia (bolder black line) are shown in the map on the right.

Spatial Accessibility Definitions

Walkshed-based measure (15-minute walking access to testing)

For each testing site we identified the area within a 15-minute walking distance (the walkshed); this process is displayed for a single arbitrary testing in the maps below. We used a national street network dataset (StreetMap Premium 2018, available with Business Analyst, ESRI, Redlands CA) to construct the walksheds. While this dataset has some information about barriers and pedestrian access, it does not include local details about barriers and access which will affect the accuracy of the walksheds. The map on the left shows the testing site as the blue dot, the 15-minute walkshed around this testing site is shown with a blue line, and the neighborhood boundaries (block groups) are shown with thin black lines. Using these walking areas, we can determine whether a neighborhood is within a 15-minute walk of a testing site. We identified any block group with greater than 25% of its area within a site walkshed as having high spatial access to testing. The map to the right shows “high spatial accessibility” neighborhoods shaded in dark gray.

15 min walking area around a testing site