Breaker one nine, this here is rubber duck

[Arlequín]

I wasn't thinking 28mm, the board would be too small unless you used a rolling road board format. 

Indeed, as the vehicles will be moving at relative speeds, you only need to represent the varying distances between them and not the distance they have both actually travelled.

I imagine there would be a 'sweet spot' between visual appeal and the representation of distance (i.e. ground scale) though... which would of course be in the eye of the beholder.
 

[uti long smile]

Pete Gill and the RAF lads were running a mad minion race game at The Other Partizan which might work.

I had a look at a few different rule sets and thought Axels and Alloys was fun - but not sure if it would suit what you are planning.