While Tower 501 and Battalion 581 were clearing from a box alarm in Gainesville, Box 16-30 was transmitted for an auto accident with people trapped on Prince William County Parkway. Engine 516 arriving first confirmed 1 person trapped. Tower Ladder 501 and Rescue 501 worked together to remove the person trapped. A helicopter was requested brining Engine 501 to set up the LZ on the parkway. All units worked well with each other. Units ready at 1207 hrs.