This weekend I drove to work on a Saturday night (when, in my part of the city, there is almost no traffic). Total commute time: 17 minutes (15 car, 2 walking to office).
I also had my shortest timed transit commute ever a couple of days later (about 33 minutes, roughly half of which was walking).
So to sum up: car commute has ranged from 17-26 min, transit 33-50. So there is a fairly consistent 2-1 ratio: the best transit commute is twice the length of the best car commute, the worst transit commute is twice the length of the worst car commute. Your results may vary.
Since the transit commutes nearly always include about 15 minutes of exercise and some reading, I think on balance I am better off taking the bus when I can (i.e. when I am not (a) in a hurry to get to work or (b) combining going to work with other errands).
Your results may vary.
