As a teenager on a 10-speed, I hit 56mph coming out of Brown County State Park. Never even approached it since.

One time, on my present bike (and, coincidentally, one time on each of the three previous), I've broken 30. It's not a matter of nerve, I'd go faster (probably start to get nervous >40), but I don't have the engine anymore. 28 is about my max, these days.