make a sponge cake in a rectangular tin. Cover with green buttercream.
Buy some sugarpaste and colour with a bit of black colouring (paste colouring better but liquid would do as you only want pale grey)
roll sugarpaste out and make a track round the top of the cake
add real 'Cars' models to the top. YOu could add flowers, rocks etc to base of cake made out of sugarpaste - or sweets, and a flag finishing line - cocktail sticks pushed into cake with paper stuck on.