You hadn't seen that sign before? Damn, they've been on the Beltway for nearly 20 years.
Anyway, they are warnings to motorcyclists (there's also a motorcycle icon with the sign). The bridge expansion joints are in the center of the lane and can be hazardous to crotch rocketeers. The joints use to be along the lane divisions but the lanes were reconfigured when the Beltway was widened.
Yes, the signs are very popular with potheads. Go figure.
