Yeah, MUNI(cipal railways) of SF, which uses a kind of honor system for buses and above ground LRVs, have a fare evasion rate of 15 to 30%, based on MTA estimates.

http://missionlocal.org/2009/08/muni-fare-beaters-slapped-wi...

Local activists typically lobby against enforcement as they interpret it as targeting a vulnerable population. On the other hand, MUNI is running a deficit.