FWIW. Not a ski area, but I used to sled in an old runner sled down Lickskillet gulch as a kid, which is a steep, quiet dirt road from Gold Hill to Left Hand Canyon West of Boulder. It's about a mile long and pretty consistent grade. A few switchbacks, a little steeper in places, but mostly long curves.
It's the steepest county road in the US, so it doesn't get much vehicle traffic in the winter. Unlikely spot for LEO's as well since I expect there to be some rule against it now given it is Boulder.
It does get plowed sooner or later, so with one of those sleds you may want to go right after a minor snowfall ( 3" - 6" ? )
We slowed down by hanging more and more off the back, or throwing a leg over the side to create more drag.
Otherwise, we also used to tow those same sleds behind 4WD. Sometimes multiples at a time and the winner was the last one sledding.