Some larger cities have repeaters set up; or at least that's what I've been told.
Yep, here in Montreal, I was pleasantly surprised as my radio came back "on" as I was driving through a tunnel. Depending on your unit, you might have an "antenna aiming mode" and it should give you the signal strength of the satellite signal and the repeater signal.
As already mentioned, your satelite signal can be easily blocked by a structure overhead (such as a bridge) or beside (such as a building). However, GPS, seems to be more resilient and will not lose the singal as easily (although, I don't think it will stand up to a garage).