I have an terrible fear of heights. I ride the Canyons here in Utah, but the ones near me I'm OK with. Rocky Mountain National Park in Colorado, oh boy, never again. I'm not too bad if there are barriers, but steep drop offs with no barriers and I'm not going that way.
I know, for a grown man to admit to this it feels stupid, but that's just the way it is. I get all sweaty, giddy and feel both sick and like I'm going to pass out. I can't do ski lifts, gondolas or roller coasters. Even going right up to windows in tall buildings makes me distinctly nervous. Having flown millions of miles in my lifetime, I'm still terrified of flying. It's all completely ridiculous but it's got worse as I get older, not better.
Riding fast on two, three or four wheels on such roads with no barriers and steep drop offs, not happening.