It is unfortunately more complicated than that. First, the density of a city is different than the density of the area over which people live who commute to th

It is unfortunately more complicated than that. First, the density of a city is different than the density of the area over which people live who commute to that city. Even American cities with comparable density are not necessarily as dense on that apples to apples comparison. Second, you have to look at commute distance. The average American commutes around 40 miles each way. The average European commutes around 9 miles each way. 4x the distance = 16x the area, which means that adding a bus from A to B is likely to be helpful for about 1/16 the people. And that, right there, is going to explain a HUGE difference in ridership on mass transit.