This map has been tweeted at me from many directions today.
You select an area and it will show you what mode: walking, biking, transit, driving; will get you to which parts of the City fastest.
You only have to click on an area in the inner NE or SE to see why cycling is doing so well there.
But the really interesting information to me is how seldom transit is the fastest answer. It only wins for medium-long trips with a direct connection.