Of course we came home and looked it up online, but all we found were videos of stunt drivers doing stunt driving stunts. It seemed unlikely that a small town would need to expressly forbid such a thing and post it on signs along the main street. There is supposed to be a YouTube video below, let's see if it works!
Eventually I remembered to ask someone when I was in town. You see, Stevensville (the State of Montana's "first permanent settlement") has such a wide main street that there is diagonal parking on each side of the street; so in Stevi (as the locals call it) J-turning is pulling into a parking spot from the opposite lane of traffic. When I drove into town after the last heavy snow a few weeks ago and saw this 18" pile of snow down the center of the street I thought, "Try to J-turn now, suckahs!"
Eric saw a 4' pile of snow between lanes of traffic in downtown Missoula around the same time. Hoo boy.