The whole thing seems like a hammer looking for a nail. It's clear from every study quoted that there were always multiple factors that changed the results, and

The whole thing seems like a hammer looking for a nail. It's clear from every study quoted that there were always multiple factors that changed the results, and they were trying to find one "overall" factor, but even that was limited. For example, most of the "walking speed" measurements are done in "downtown locations". Most cities are not made up of downtown locations, downtown is one location in the city, so the measurements only indicate why people walk fast in downtown locations. They also quote other factors that change the results, like environment, and culture. So basically the results change for any cities that aren't identical. And they're trying to use a national metric (GDP) to relate to walking speed in individual cities, when it's obvious that walking speed is going to relate more to local economic metrics, not national.