At about 0625 Pacific time, a Cessna 172H struck the median of Interstate 5 about a tenth of a mile from Weed Airport. The airplane was unoccupied at the time and no one on the ground was injured. The pilot said the battery was too low to start the engine, so he attempted to hand prop it. The engine started and the pilot fell to the ground, sustaining a minor injury. Thereafter, the runaway airplane departed in a northwesterly direction and flew several hundred feet until crashing into the ground and nosing over. The pilot said the airplane was not secured in any way. He also said he thought the throttle was set at idle speed but later realized he had left it at full throttle.