I would become your best friend in the world if you built it, but never call you once the townsfolk caught you doing construction and had it stopped. My friend's friend (there was an article, so this isn't just heresay) began to build a motorcycle track on his private estate, and I believe even got the proper permits to go ahead with it. He was about half way in to the building when the NIMBYs (Not In My BackYard) filed and got an injunction to halt construction due to "potential noise pollution" caused by such a facility. The bottom line is that it got very very very very very messy, even after he went through all of the proper channels and thought he was, and should have been, OK. I'm looking for the link to the story, but can't find it.
IF I were going to take the risk and try it, I'd build a karting track, save yourself some money, and have A LOT more fun. There's nothing like doing 120 mph in a Kart that is an inch off of the ground, and then turning on a dime. The famous quote is along the lines of "about as much fun as an F1 car, for a few million dollars less."