Search in SP's threads, there's one here somewhere that gives a formula for estimating force of wind on a standing human. As I recall, at my weight and size a steady 95mph would be enough to get me airborne if I leaned into it.
To give an idea how imprecise the numbers are, Wikipedia tells me that the terminal velocity of a falling skydiver is around 120mph - that's in belly-down orientation, and I assume for a standard-weight male. In other words, dive into a vertical wind tunnel blowing upward at ~120mph, and you'll float, immobile, as long as you maintain maximum area facing into the wind.
But getting knocked off your feet requires a lot less wind than staying aloft would require. Gusts add to the challenge, of course. As a rule of thumb, anything above 73 mph (hurricane speed) is going to make walking very interesting.
Here in New England, we have the advantage of a weather station on the top of Mt Washington. Just walk around on the summit, take your best guess, then go inside to see what the actual speed was. Or since the door is closed in winter when the big winds come, check online once you get back.
I generally check the forecast *before* heading up, so I can't recall going past about 60mph. At that speed, it's like crossing a fast stream: you have to pay attention, and you don't want to be carrying anything remotely kite-like (like snowshoes; even hiking poles will drag on you), but you can walk pretty normally once you adjust to leaning a little.
Speaking of the Mt Washington observatory, here's a good video of 86 gusting to the 90s:http://www.youtube.com/watch?v=czazfwci ... ure=relmfu
That poor guy has no axe, nothing to grab or shelter behind, and is not wearing traction. The same site has videos of folks doing various stuff in steady wind in the 70s, and while they're not exactly graceful, they're pretty much upright.
Example: http://www.youtube.com/watch?feature=en ... C3DUwseZn8
So I think you underestimate humans' ability to handle a good blow. But I share your skepticism regarding uninstrumented estimates. Most people have no way of calibrating their wild guesses about wind speed.