there doesn't seem to be any reason why he would drive off the road and hit a fence post unless he was going too fast.
Just one last comment, Lizzie. That's entirely possible, but having lived and driven on the upper reaches of the S. Deeside road years ago, I'd suggest another possible explanation. It's a rural road, and would have been even more so in 1926. There are rabbits/pheasants/deer, etc., all without a shred of road sense, that run out unexpectedly. Swerving in a car is dangerous; for a bike it can be catastrophic. The culprit vanishes unscathed, and all that is left is another apparently inexplicable accident. I'd give the poor man the benefit of the doubt.