Without looking it up: what percent of households in Madison don’t own a car? What percent of commuters don’t drive to work? What percent bike?
My guesses were pretty far off.
The answers, along with similar data for other cities (drawn from the 2000 census) are here.
Quomodocumque 
But maybe people who don’t own a car are more likely to respond to the census long form?
Possibly the numbers for Madison are skewed somewhat by the undergrads? The census normally counts students at college (rather than at home), so there’s an extra chunk of non-car-owning (and non-commuting) folks in town.