That's quite the limit. That's one hell of a limit. Most people aren't planning on going 40-60 MPH in a bycicle.
From wikipedia: "When considering human-powered equipment, a healthy human can produce about 1.2 hp briefly (see orders of magnitude) and sustain about 0.1 hp indefinitely"
A 1HP DC motor (or if you want to get crazy with speed control and all that jaz you could do a 3 phase system with a PDM from a torq-controlling pedel or something) is more then enough! Hell we don't even need that. Let's think a .5 HP 1 phase 60Hz motor. This clocks in ~800 watts at continuoious max current but we'd probably not be doing constant drive on this. But let's say that we can pay a little more and find a perfect 750W motor for this at .5 HP. That's a lot of torque given that most people can only sustain .1 HP. This motor (which is crazily oversized by the way) is rated for .5HP @ 3450RPM. 3450RPM at the chain is different to the gear ratio at the wheel. The lowest gear ratio on a bike is about 1.9:1 so our motor would be putting out quite a bit of thrust and even at the reduced torque you'll be halling ass.
Now this is a super oversized motor but anything that can take 750W continuious input is some serious shit. That's one hell of a motor and to complain that that is too small? Get a harley and tinker with that man.
[0] - http://www.ebay.com/itm/MARATHON-1-2HP-AC-MOTOR-M400095-208V...