The pilot reported that he departed for a parachute jump flight with 12 gallons of fuel onboard. He added that, after the parachute jumpers exited the airplane about 10,500 ft mean sea level (msl), he initiated a left spiraling descent back to the airport. He further added that he "heard and felt the engine start [to] quiet down as if it was shutting down." He then began to make right descending turns and verified that the fuel selector was in the "both" position. He added that the cylinder head temperature was decreasing, so he switched back to left descending turns and that the "fuel starvation due to banking happened two more times." The pilot reported that he entered left downwind about 4,000 ft msl, pushed the throttle and mixture controls full forward, and determined that the "engine wasn't producing much power." He added that, during short final, he realized the airplane was too low, so he landed the airplane on a highway "on-ramp." During the forced landing, the airplane impacted a guard rail and a post. The airplane sustained substantial damage to the fuselage and both wings. The pilot reported that he "suspected engine power loss due to fuel exhaustion." During a postaccident examination, the Federal Aviation Administration inspector drained about 12 gallons of fuel from both wing tanks and the gascolator. In the Description section of the Cessna 182A Owner's Manual, it states that there are 1.5 gallons of unusable fuel per fuel tank (3 gallons) and that, when not in level flight, there are an additional 3.5 gallons of unusable fuel per fuel tank (10 gallons).