If your question is really, should I buy a car? The answer is yes.
For family med, I was one of the people who lived "closest" to Racine (40 miles away), drove down there every week day -- clinics not hospital. I liked the rotation though.
For OB/gyn I was by the lake. For Psych I was about a 30min drive from the school. For peds outpatient, I'll be by the lake again. During medicine, I'll be at the VA 15-20min from school depending on traffic, and St. Joe's absolutely no idea where that is, but not walking distance.
Edit: I guess they could be walking distance if you got an early start.
Edit pt deux: You probably don't want to do ALL rotations at Froedtert. I was there for 4 months straight and wanted to shoot myself by the end (ok, well, maybe not that bad, but I hated going to work every day and wanted to claw my eyes and pull my hair out several times). It's the busiest place you'll be. It's good for rotations in which you're interested, but it's also a GREAT place to avoid if at all possible.