So, I've spent the whole day trying to learn how to use SketchUp. All in all, I'm quite pleased with the result - especially that I've never used the programme before. To keep things nice and simple, I started by constructing a one-mile banked oval course (15 metres wide, with a concrete apron on the start-finish straight), before adding a neat little infield loop (12 metres wide) for small-time road races. The pits have 26 garages, each of which also has an upstairs viewing area for guests and VIPs.
I also added curbs and gravel traps for the infield area - the gravel comes right to the edge of the track, and would severely punish anyone who takes liberties with track limits. On the infield side, the safety barriers are about a metre high so that marshals can get to incidents, while on the outside they are two metres high with a further catch fence above in order to protect the audience from any debris.
I'd be interested in any feedback you can give me, because this is my first time and although I won't be using SketchUp very often, I'd still like to learn.