Until Nixon's National 55 Oregon's primary speed law was the Basic Rule. As interpreted, there was no set upper limit, the driver was responsible to maintain control . That law is still in effect but since the 55 posted limits have been given priority status. For you east coasties, its probably hard to imagine a modern 4 lane freeway stretching more than 100 miles in areas with less than 50,000 total population.
I ve never driven east of the Rockies but the fastest traffic I've seen was in Arizona. Ninety mph was right lane speed.