Thank you, Jamieson.
From that map, it doesn't look as though, nowadays, there is an actual junction where the roads meet, but I've looked at it on Streetview so at least I know more or less where he was killed.
When I googled South Deeside Road, I found a few articles about recent car accidents where people were killed along the road, so I guess it's still a dangerous B road. I imagine people just drive too quickly along it. Whether that happened to my grandfather we'll never know. A newspaper report at the time (which a kind Rootschatter found for me a few years back) states "How the accident occurred has not yet been definitely ascertained, but something had gone wrong with the machine, which swerved and crashed into a fence at the side of the road".
Lizzie