Do a quick google image search of horse trailer ramps. It seems they're usually rubber with ridges of various sorts to get traction. Plus, it probably won't freeze as quickly since it's rubber, but being in the New York area, you're going to have to constantly put salt down to keep it from becoming icy.
That'll also give you a good idea for steepness, though I'd also go with the ADA recommendations.
I think you should definitely put up railings the entire way - the last thing you want is a spooking horse (and/or it's person) to suddenly fall off because it wasn't watching where it put its feet.
Take that into consideration when designing width, too. I, personally, would hesitate to take my horse up an long alleyway (which is essentially what you're making) if I couldn't turn him around inside. I'd hate to back my horse down something that long if something happened, and no matter how narrow, a horse may decide he's turning around if it's the last thing he does. I know a few that'd take out the sides/rear/go over backwards if they decided they wanted off that ramp. And of course, something will ALWAYS happen sooner or later. Imagine if the invisible boogyman in that specific corner of the arena suddenly joined you on that ramp?