It is because modern urban areas develop in a dispersed and decentralized pattern, and it is impossible to design transit systems that can effectively meet such dispersed demand. For example, very effective transit service is provided from the suburbs of Paris, where 80 percent of the population lives, into the city. But traveling around the suburbs of Paris by transit can be as inconvenient as in Denver. Auto competitive service is simply not available for most trips. It does not take great intellect to understand that more of something in a constrained space increases crowding.
Traffic volumes are four times as high.
It is true that the average amount of auto travel per person has declined, but the decline has been small, and the overall impact on the community has been to significantly increase traffic congestion, and by extension, air pollution. Traffic congestion has intensified rapidly. The Texas Transportation Institute's Roadway Congestion Index for Portland is nearly the same as in Atlanta with its legendary traffic. Portland has recently been found to be in violation of air quality standards. And housing prices have skyrocketed as a result of forcing all development within the urban growth boundary.