I have Bobs Track Builder and I have been messing around. Unadilla motocross track has the Gravity Cavity and thats what inspired me to make this. Let me know what you think before I release it. Or if you want to test it I can post it here. The total track run time is about 9 minutes. The Video you see here is only 1:30 about. Excuse the sound I have not explored how to record straight from the computer, for now I just point my camera at the screen.
I like the idea and I would like to test it. Why don't you try to make a version with the road half the wide it is now. It will be more narrow, the stage will become very tricky with those crests and jumps and it will be a lot of fun in my opinion.
Try the program Fraps for capturing video with sound (find it with google). It is very easy to use