I bow to your superior memory Philip. The point I was trying to make is that there were signs. Even in Montana. And there were speed limits. Again, even in Montana, unless you were driving a car in the daytime in non-congested areas.
The highway signs in the mid-90s had Reasonable and Prudent posted as the daytime speed, 60 is posted for trucks and 55 for the nighttime speed for all vehicles. Of course there have always (well nearly always) been speed limit signs posted for congested areas, such as towns, school zones, etc...