On September 22, 2002, about 0625 Pacific daylight time, a Cessna 172H, N2469L, impacted the median area of Interstate Highway 5, about 0.1 miles northwest of the Weed (uncontrolled) Airport, Weed, California. The airplane was substantially damaged. The private pilot sustained a minor injury. The private pilot, who owned and operated the airplane, reported that he intended to make a personal flight under the provisions of 14 CFR Part 91. His destination was Fall River Mills, California. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot stated to the National Transportation Safety Board investigator that when he attempted to start the airplane's engine to initiate the flight he noted that the battery power was low. The propeller would not rotate. The pilot then exited the airplane and rotated the propeller by hand in an effort at starting the engine. The engine started, and the pilot fell to the ground when the airplane jumped over its chalks. Thereafter, the runaway airplane departed in a northwesterly direction and flew several hundred yards until crashing into the ground and nosing over. The pilot further stated to the Safety Board investigator that during the hand propping operation the airplane was not tied down or secured. The pilot also indicated that he believed the engine's throttle was set near the idle position; but he subsequently realized it was at full throttle.