A friend of mine is working on this project. Really cool: a "flow trail" in Santa Cruz. Honestly I thought the US was way too litigious for anything like this to come together, and there was too much anti-bike sentiment among California NIMBYs: I thought you had to cross the border up north, to Vancouver maybe.
They're competing for a grant from Bell Helmets. Consider voting for them. Their project page is here.
Rumor is we may be able to ride it in the opposite direction for Low-Key Hillclimbs at some point.
While you're on the Bell site, make sure to also check out the video for the Stafford Lake Bike Park.