Mystery Dubya

Another crappy camera-phone picture, this one of the back of a Houston Metro bus. I have noticed that most (if not all) of them have this "W" in white on the right rear. Why? What is it for? Anybody know?
I sure hope it's not a "W, da prez": What next, larger-than-life statues at major intersections just begging to be toppled by uninvited, unjustified invaders?