Even Dash with his super speed can't use the quarter pipes, stunt ramps or loops. He just runs into the walls and bounces (at least he does for me).
The cars use them more like a wall to do a u-turn and some stunts and it isn't as easy to get them to go beyond the ramp. This limitation convinced me to use a standard hill or slope in place of the ramps.
Cars are the only characters that can successfully use the loops but this is essential and I can't remove the loops. Characters can simply go around the loops.
As for the path being narrow, I am thinking about using topiary to restrict the back side of the path. I haven't done much with them yet so I am not sure how viable they are as a barrier.
I think this will be a challenge but I hope it turns out!!!
Four Featured toy boxes