[AndyJudge]

Airfields would work but would need careful marshalling, I know the Scooby event at the Prodrive test track failed as the front of the convoy linked up with the tail of the cars waiting to take their places.

Maybe the Cavalcade could use the runway as being the point where measurement is taken (about 1/2 way down) & then the perimeter track & taxiways to form up on before joining.