My personal fastest is 83 km/h (about 51 mph) coming down out of the Gatineau Hills in Quebec. Scary as hell, and you can imagine how shocked I was when 3 other cyclists went flying past me, probably doing well over 90.

I used to do an organized century ride from Ottawa to Kingston, and there was this one hill with a speed limit of 50 km/h that cyclists could easily hit about 70 on. The provincial police used to park a cruiser at the bottom of the hill to deter cyclists from going all out and flying into the town at the bottom of the hill. I heard that a few cyclists were ticketed the first year the cops were there, but never knew anyone personally who got a ticket.