La Renault Clio V6 Music 10/10
Gameplay 5/10
The "game feel" is greatly enhanced by increasing the frame rate. (Why did you pick 20?)
I found good results increasing the frame rate to 30 fps.
I also tried 60 fps, but then it is necessary to lower the car's max speed to compensate for this. (Even better would be frame rate independent movement, but that is beyond the scope of this comment.)
Another user mentioned collision. AS2 has a method for point-shape collision, so I think the best bet would be to check for each of the 4 corners against both of the grass shapes.
This has potential! I think the map should be a bit bigger. Maybe add a camera? The math can be a bit confusing (doing it from scratch) if you haven't done it before, but I think there are VCam scripts that make it easy.