What you need is the hill across from my place. It's a road that they don't plow for almost 3 miles. ALL of it downhill!!!
My wife and the rest of the kids in the area used to ride sleds down it a lot. The hill starts off rather shallow and then drops really quick. In a car you can end up at almost 60 mph just coasting. On an icy slope with a runner sled I would bet the speeds would be the same or faster.