I have just read all 5 pages of posts on this, and one thing is missing from all of them. What I have noted is:
1 The road, apparently a flyover, had a gentle curve to the left, assuming that the quad bike was travelling towards the camera in the main photograph in the OP.
2 The surface appears to be in good condition.
3 The weather appears to be fine.
4 The main damage is to the offside front wheel of the quad bike.
5 Given the things listed above, even with the lack of grip of quad bike tyres, there was no sharp turn so he should have been able to manage that gentle curve without skidding.
6 There is therefore another possibility, and that is that his death was the casue of the accident rather than the result of it, just as happened with my friend Colin Hastings in Jomtien 11 months ago.
7 Assuming that he was coming from the top right in that original photograph, towards the camera, he appears to have travelled in a straight line, croosed the central line markings into the opposite lane, and carried on to hit the barrier. It is therefore certainly possible that a 62 year old had some sort of heart attack, stroke or seizure, failed to take the gentle curve left and went in a straight line hitting the barrier. The amount of blood on and around the body would give an indication, but there was no reporting. Hopefully an autopsy, as with Colin Hastings, will clear it up, but then most of the posters on AN will have to eat their very cruel words. RIP Scotsman.