While returning from a commercial AFA on Rockledge Drive, Tower 751was stopped at a traffic light on Olde Georgetown Road.  Members were patiently waiting the green light when a pedestrian was struck by a vehicle in the opposite lanes.  Captain Forrester called in the incident to communications while Master FF Lowery dismounted the Tower to prevent the fast approaching traffic from striking the victim again.  Master FF Smith positioned the Tower to protect the scene and the crew initiated patient care until the arrival of the Bethesda Chevy Chase Rescue Squad.