Not sure that's quite true. They had "reasonable and prudent", IIRC, which meant that a cop could still write you a ticket for 140. If there was much other traffic around, he might have been justified in doing it.
Also, I get it for eastern MT, but my mom and step dad were living in Helena at the time, and my step dad drove way too fast for those unstraight mountain roads. It wasn't just traffic that they brought the speed limits back for.
