I operate from a fixed area. I couldn't imagine broadcasting mobile.
I hear ya atrain, but portable operation using pre-recorded programming works quite well and is a great way to keep out of the Fed's grasp. Back in the 80s when I used to play with FM (statute of limitations, nah nah nah!

), one of our favorite ploys was to drive up to the rooftop of a downtown parking garage, plant a 1 watter with a whip and a portable cassette deck with auto reverse by one of the lamp posts. Everything ran from a couple of 6 volt lantern batteries wired in series. We would leave it running the same 90 minutes of programming until the batteries ran down, which would be about 18-20 hours. Then we'd retrieve the package a couple days later. The one thing about the FM band is: few people tune around on it. If you aren't on for long long stretches, or run a regular sked, no one will hear you. This way we could saturate the downtown area of the city and be on long enough to be noticed.
Nowadays, there's a LOT more cameras around to avoid and everyone is FAR more aware of things like unattended bags laying around than they were in the 80s. BUT - Tiny MP3 players and tiny FM transmitters - both of which are able to run on much lower power requirements (much smaller battery pack) make this form of operation still possible, given a little creativity. The whole point of this type of operating atrain, is to reach a lot of casual listeners - not just the dozen or so Buddies that listen for your signal.