A car accident theory makes all the sense in the world to me....nervous, tired, stressed out, a curvy mountain road....all things that would seem to put you at an elevated risk for an accident. The only things I can't reconcile with an accident are 1) the fact that her very red Jeep has not been found, and 2) if there was an accident, I would think it's likely that her phone would have been somewhere in the vicinity of the accident when it last pinged. So even if her Jeep had rolled out of sight somewhere, the phone would at least have narrowed the search area. Yet we haven't found Gail. In addition to organized searches coming up empty, no hunters, off-roaders, bird watchers, curious kids, Roy Kronk types, have found Gail, the Jeep, the phone, or anything else that could be associated with Gail. The longer that pattern continues, the less and less likely an accident seems to be, although nothing can be ruled out.
I'm not local and have zero inside info, but I have an active imagination and sometimes, intuition. Having read about the mountainous roads in TN and the possibility that she was heading to AL or even NY (long drives) or wherever, it just seems too possible that she could have been followed/intercepted and run off the road. Her red Jeep should be very visible but as many have posted, the summer foilage may conceal it or it may be underwater. Or it may be otherwise hidden (nefariously...stowed somewhere, burned, chopped, etc.). 
