This is a bit dated, but it comes from the Metro Source blog:

The Source » The trains are always faster on the other side – other cities look to Metro for inspiration

What’s the point? Well, if statistics are to be believed, people who commute by transit, even in places where the transit is good, spend more time commuting than do those who own cars. Now, the truth is we don’t necessarily want to make too much of these differentials at low levels: anything below 20 minutes is a bit of a whatever, and I’m not sure people even think about differentials at such low times.

But the kicker with transit commutes, I suspect, is not the average but the standard deviation and the skew in the distribution. The only thing that disrupts a car commute is car trouble or traffic. Transit, from an individual perspective, has these problems plus the uncertainty of your arrival time jiving with the service, etc. And the only thing that vitiates that is service frequency.