As the evening rush hour was commencing units were dispatched to the reported MVA in the 12600 block of US59 southbound near S. Kirkwood. While enroute SPD units advised of the incident being northbound and involving a subject thrown from the freeway overpass. Upon arrival of 2104 one male subject was found along the grade level feeder suffering a lower leg amputation and other major trauma injuries after being 30′ from the main lanes above. With Engine 23 arriving seconds later the crew applied a combat tourniquet and began trauma care while requesting air medical. Upon arrival of Battalion 21 command was transferred, Truck 21 requested for the LZ and Rescue 21 directed to the main lanes above. With the primary patient in the care of Engine 23 and FBCEMS Medic #3, Rescue 21 and FBCEMS Medic #5 packaged 2-additional patients on the overpass and then secured a diesel fuel leak. Upon the patient being loaded with the PHI aircraft and safely departing units assisted SPD with traffic control until released.