Reuben Fischer-Baum analyzes monthly ridership data from the National Transit Database and population estimates from the American Community Survey to calculate 'trips per resident" for 290 urbanized areas. The metric allows a comparison of transit trips between cities of varying sizes.
One of the larger questions examined by the article in reaction to the data follows: "Do certain regions tend to support better public transit, or is this just a product of city size and density?" Here's how Fischer-Baum responds:
"Among all 290 cities, there’s a clear relationship between trips per resident and both total population (the r-squared is 0.41) and population density (r-squared = 0.21). This means that it’s not particularly revealing to map all the cities together, because larger, denser cities are clustered in the Northeast and on the West Coast. But if we look only at the 248 urban areas with fewer than 1 million residents, the total population relationship (r-squared = 0.0002) and population density relationship (r-squared = 0.07) disappear."