Not very much can be ruled 100% out. I don't think there's even a consensus here on what happened.
The major problem with any "hit and cover up" theory is that it would have happened after Jason disrobed (since the clothing showed no signs of damage, other than a red blood spot). After the crash, and three minutes after he started walking down the road, and for unknown reasons, he disrobed, with the order of clothing found that suggested he was walking fast/running as he did so.
The odds of someone deciding to "panic and cover up" after hitting someone are very low. Without trying to get excessively macabre, I think it falls to 0.0001% that someone would try to "panic cover up" after hitting a naked person on the road.
From what others have posted, Salt Flat Rd is bumpy and potholed. Nobody is going fast. (It was described as: if Jason was being chased at a decent speed, he would have blown out a tire long before he reached that crash site.)
At the moment, this is the major question that bothers me: in which direction did he go after taking his clothes off? As he took off his sandals, I'd think that his feet must have gotten cut up/there has to be a trail somewhere. At the moment, I believe it that, if there was a trail, it was ruined/made unusable in the hours and hours afterwards.