The first time I saw this here in Edmonton it was just one company. Now I see several companies in this area, and they seem to be sticking around.
There was one that used a Smart Car that was pulling a two sided billboard where the advertisements changed every 5-10 seconds. that was a real eye grabber.
Look into Billboard Connection:
http://www.franchisebrief.com/franchises/billboard-connectio n-franchise.php
They are a franchise and I now they are doing pretty well.