I chicken out at 40MPH. That is the speed where my mind stops thinking about the road and starts wondering about fork failure, a sudden flat, a car pulling out, an unseen pothole, a patch of gravel on the shoulder, a pack of javalina running into the road (or coyotes, take your pick), etc.. I suppose if the road were really smooth that number might rise a little but I have yet to find a smooth steep hill around here. BTW I hit this barrier at 100 on a motorcycle. Guess I am a lame- o. Perhaps this is because of past crashes. Luckily I have only had sprains and not broken bones, I did hit a car going 30 when I was young and more stupid (my bike racks were full of newspapers which exploded everythere when my rack clipped the car which suddenly stopped in front of us). I ended up flying through the air over my handlebars and landed about 15 feet down the road. My leg caught a truck bumper on the way.