No Shuttle Buses Allowed on the Shuttle Bus

I saw this sign on the airport shuttle bus on my last trip (click to enbiggen). The first symbol is clearly "No smoking." The second symbol obviously means "No food or drink." That third symbol doesn't make much sense at all. I assume that it is supposed to be "No radios," however why would they use a image of a console radio to represent that? To me, it really looks like a shuttle bus. If they have had this problem in the past, prohibiting people from bringing buses onto the bus is probably a good idea.