If you mean a universal legal maximum speed Nigel, I think 20 mph, since I don't think all manufacturers take their design safety responsibilities seriously enough, some just wanting to shift "boxes" for profit. I also base that figure on the fact that in general terms, nature has designed and evolved the human body to survive the consequences resulting from what it's naturally capable of, and that's about the top speed a fast runner reaches. This is also the basis of calls for all town speed limits to be 20 mph.
If you mean personally for me as an individual at all times, none, since I prefer to make my own judgement on a bike's capability and the riding conditions. Unfortunately, many people feel the same as this, even those who's judgement and experience is seriously lacking! Even the ultra light bikes used in the Tour de France and other racing reach 70 mph downhill, and recumbent bikes with "ordinary" riders can reach over 75 mph downhill. Also, the world speed record on the flat for an enclosed unassisted bike and rider is 83 mph. (Canadian Sam Whittingham riding a Varna Diablo designed by Georgi Georgiev)
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