Originally Posted by oldgoat
Seems to me ski areas have been marking trails for difficulty in a fairly standardized way (some use double blues, some don't use double black but mark expert slopes with an experts only sign) since well before anyone passed a law about it.
Makes sense to me that the legal jargon came well after there was already a standard in the U.S.
Here's one article that explains the American system, with a little history:
A Little History
The National Ski Area’s Association (NSAA) first adopted a system of signage in the winter of 1964-65. Before that, as if skiing on wooden boards with leather boots wasn’t hard enough, it was every man for himself. The colors they used were different than today and also conflicted with the colors used in Europe. American Resorts used yellow to designate intermediate trails and the French used red because, well who knows why—they’re French. The Walt Disney Company was considering opening a ski resort and did studies and tests on which colors worked best. They came up with the current green circle for easiest runs, a blue square for intermediate, black diamond for advanced and double black diamonds for expert. This system was adopted by the NSAA 3 years later in 1968 and has stuck ever since.
NSAA has a webpage that explains the American system. Includes a link to an article in Skiing History from 2012 that has a little more info about the Disney connection.