Rockville, despite being a fairly affluent area, is unfortunately in Montgomery County.  The cops are known for going after this sort of thing.  The vagrants seem to be allowed to practically accost you at any intersection when you're stopped for a red light, but resources seem to be towards big busts of Asian massage spots.  I assume hotels are more vigilant too....  otherwise, you're better off driving over the bridge towards Tyson's Corner.  Do it early, as the traffic will make you become a raving nut.