I have hit 51mph on a local rolling hill loop I use for the purpose of fast runs. Regarding the claim that you would know if you hit 50 mph, I notice my speed as it gets into the 40s generally but I then am paying attention to bike handling and checking the top speed in the download later.
__________________
My stable:
2013 Redline Conquest CX
2010 EasyRacer GRR
2015 Cruzbike Vendetta
2009 Raptobike Lowracer #0067
My blog
My bike pictures