I am not sure they should scale any worse than any turnpike rest-stop with food, the average time any person parked there stays there would be about the same (and chances are, people are going to want something to snack on while they wait). (On that note, I heard an idea in another HN thread about this that certain small businesses may provide charge-ups for free to customers. The power cost would be negligible (installation a bit more of an investment), and currently it would attract a likely profitable demographic. This variation of the idea should certainly scale.)

The one problem I see is if you could actually get enough power to a place like this the size of a parking lot.