The walkscore data is separate from the earnings records; it's calculated almost on-demand from the Google Maps API. The earning records are from https://opportunityinsights.org/paper/land-of-opportunity/, they didn't actually analyze the records themselves. And yes, 382 isn't particularly fine for geography of the US: https://opportunityinsights.org/wp-content/uploads/2018/04/m.... For comparison, there are 3142 counties and they could have used that data instead.