Well ... if you want Steve McKinney to move at any speed faster than stationary, you'll have to dig him up.
There are at least five different discussions going on here:
1: Putting a mph number to your own speed, or your friend's speed, or the speed of someone you don't even know, but saw in a bitchin' movie once.
2: How fast you go relative to other people on the same hill.
3: How fast you go relative to other people on some other hill.
4: What skills and equipment are or are not required to go how fast.
5: How fast you should or shouldn't go.
Topic 1, if you seperate it from the others, is kind of abstract and academic. It's not enormously relevant to anything, unless you're planning to race a car and want to know how much to bet on the outcome. Not to say it's not interesting, but the interest is purely academic. FWIW, my guess is that (i) 50 is done fairly often by people on ordinary mountains when they're really moving, (ii) 60 is fairly rare, but not unknown and (iii) 70 is almost unheard of.
Topic 2 is kind of obvious to you, and to anyone who's standing there as you go by. For one thing, you can see these other skiers going past you one way or the other. Hint: if they're going from in front of you to in back, you're going faster than them; if it's the other way around, they're going faster than you. How suddenly either happens tells you how much.
Topic 3 appears to be of great interest to some people here, particularly Jer. You can get to it by way of Topic 1 or 2, but
- To get to it from Topic 1, you need reliable measurements, in mph, of various people's speeds. If you simply say, "I ski much faster than you, so I must be going 70 when you're going 50," you're reversing the logic and assuming the answer in order to determine the measurement.
- To get to it from Topic 2, you need to know how the "general" speed of the skiing population at different areas differs. I don't think this is readily knowable, though it may be "obvious" to some the the general speed at Alta is much higher than at Killington because ... just because! Actually ... I'd say it scarcely differs by region, except where the size of hill restricts it, or there's a very skewed population.
Topic 4: Skills - ability to stand up while going down hill. Generally learned the first day. That'll get you to 50 at least, if you're stupid enough to do it on a very steep hill. Do some version of a tuck in which your shoulders aren't way higher than your hips and your hands aren't touching your bellybutton, and you'll add quite a bit of speed (perhaps enough to send you spinning end-over-end in a cloud of snow and equipment at the first bump).
Equipment - Skis.
From there: If you want to go faster than someone else going down the same hill (e.g. to go 65 when he's going 60) -- a really good tuck; a suit; downhill skis; perfect wax; relaxed ankles and knees; etc.
Topic 5: Take it as a given from observation that some people sometimes ski too fast. Are any of those people you? If you are always going faster than everyone else on the hill ... probably.