I get a bit antsy at low 40's, partly because I'm a forensic engineer and have investigated bicycle and MC accidents. If you fall at this speed, you will be hurt. what can cause you to fall? Road debris and defects are pretty problematic, the worst I've seen is the 1" lip on a utility trench that settled while a rider was going about 40, the front tire rubbed the lip and he went down hard, busted pelvis, pins and concussion. So, make sure if you're going to go that fast, know the road.