I live in Illinois flat as they come so on biggest descends around here 40 is pushing the limit. In the few times I have been in hilly places I hit 43.5 and thought that was pretty much my limit. If conditions and roads great I could see 50 but not above that for sure. I once biked in mountains in Montana but no really long stretches that would seem to allow the speeds some are talking about. I would like to see the Strava data on the 75 mph hit by someone. I think every once I a while a pro tour ride gets near that but no often and they are pros. Over time one can get better and use to the speed but for me it takes time given where I live.