It's a tragic accident that reminds us all that life is so vulnerable. You can never be too careful. As for the cause of this accident, I completely agree with yxie who posted the following message:

"This is a good thread. I've been thinking about the cause of the accident since it was reported. One thought that kept popping up was that it might be related to the glare from the rising Sun because the time of the accident was early morning. We all had times during sunrise or sunset when we got totally blind sighted by the Sun glare directly into our eyes.

The pictures took by Yuan's Mom TAGG just published seem to confirm this theory. She took a few pictures from 7:00 till 7:30 am, moments before the accident. From the pictures, you can see the Sun was shining right through the windshield. Yuan was not wearing sun glasses. The last picture showed him driving with the strong Sun shining through the windshield covered him below just below his nose. I would argue that when he made that left turn, he was directly looking into the Sun and blinded him. He was trying to stop the car but couldn't do it in time.

This is still a theory but with the pictures it seems to make sense."

Just want to add a few words. I have driven along 120 a couple of times, the latest in June. As many people have said, it's a very dangerous road with a lot of sharp turns. You have to keep concentrating on the road all the time while driving. It could be fatal if you take your eyes from the road for only a second. It's already a tough challenge for Xiaodong to drive on such a road when another factor stepped in: the Sun. We can see in the pictures of Xiaodong and his mom taken at 7:00 am that it's still pretty dark and the sun had not risen. Then in the two pictures taken by the mom at 7:29 the sun was just rising above the horizon (or mountains, to be accurate), right shining into the car that's going east. Xiaodong did not wear sunglasses and nor was the sun visor pulled down. The sunshine was just an inch below his eyes. I believe he was a good driver as described by his brother. But he just did not have the experience needed and was not prepared for what's gonna happen. Under a very unfortunate circumstance, Xiaodong would be looking right at the blinding high-altitude sun (~ 9000 feet, you might notice that the rising sun is brighter than it is at low altitude since the air is thinner and cleaner) so he could not see the road (probably anything) for a couple of seconds until it's too late. The rest is history.

May they rest in peace.