Collision with steep terrain

I'm using the script from the car tutorial for my own driving game. In my game the player drives off-road on terrain created with Unity's terrain tool. I've found that Unity's built in collision works great on terrain surfaces that are flat, but when I have a sharp change like on the edge of a cliff I am able to just drive right through it.

Is there anyway to fix this without having to put invisible colliders in front of areas where the player could possibly drive through?