I've been a member of ZipCar for well over a decade, from when they first came to London and issued the plastic NFC cards. Then it was the smaller ones that cou

I've been a member of ZipCar for well over a decade, from when they first came to London and issued the plastic NFC cards. Then it was the smaller ones that could attach to a keyring, and for the past 5 or so years - just the mobile app. This is very anecdotal, but I never really had any issues - even more so, the last ZipCar hire I did took me from downtown SF to the Muir Woods (no cell reception at all). I had no issues locking / unlocking the car during the trip - I'm fairly certain it uses bluetooth for the connection between app and car; but likely needs data for starting or ending a trip.