Which, if you think about it, makes perfect sense. Everybody has some unconscious limit of how long they are willing for their commute to be. Am I willing to commute for 30 minutes? Probably. What if it was two hours? No way. I'd either find a closer job or move closer to work.

Average that over the whole population, each with their own individual unconscious time limit. The result is: if commuting is faster, people are willing to live farther from their jobs. If commuting is slower, people live closer to their jobs. Adding more capacity doesn't change this.