Yes it does have a speed limit of 40 but like most rules, it is happily broken by some.
Having lived in Manhattan for 10 years I can assure that it is possible to get to 80 on the FDR and does happen.
I do not know why people are so wide eyed as to believe that this could not happen? A motorbike, moped or a car can easily get to 80 mph and quite quickly nowadays - in fact it is a selling feature of motorised vehicles.
I saw a young man walk casually behind a woman on 1st ave, grab her phone (she was texting at a bus stop) and bag. Out of nowwhere three mopeds raced to him reeving up, he jumped on, they drove away at very high speed, up a one way street. The noise and chaos were really scary. The speed was terrifying.
It is the chaos which creates the potiential for things to go wrong.