The paper starts by talking about a correlation between a city's "economic activity" and the walking speed. But the graph shows population vs. walking speed.

The paper starts by talking about a correlation between a city's "economic activity" and the walking speed. But the graph shows population vs. walking speed. Surely population and economic growth are independent (New Orleans, Detroit, etc.) so I'd like to see if anyone has done a study comparing an economic indicator like DGP of a city.