Anecdotally, the people I work with in Cambridge, who live in Boston, Cambridge, Somerville and Arlington own cars, but don't use them to get to work. The cars are for the weekends and vacations because there's a lot of leisure activities in the area that are out of reach of the T. I know a lot of people that don't ski and don't even dig their car out after the first big snowfall - They just let it sit from December until March.