For Montana (where I lived at the time), there was no posted day time limit; night time it was 55 MPH. While no posted limit, the law stated something to the effect of "must drive reasonable and prudent for conditions". Conditions here is kind of vague, but is pretty all encompassing: road conditions, weather, traffic, your vehicle, your tires, etc. Bright, sunny, summer day in a Porsche with good tires? At 100MPH, you might have gotten pulled over, your tires checked, and then the officer would send you on your way. Blizzard conditions? Driving 45 might get you cited. And the citation had teeth: it was automatic reckless driving, something like a $500 fine and possibly a suspended/revoked license. Previously, the fines were a joke. Something like $5 for up to 20 MPH over the limit, could be paid in cash and didn't go on your record.
Anyways, "reasonable and prudent" was eventually rule unconstitutional because we can't have nice things. So, out it went, and instead it was replaced with a 75 MPH daytime, 65 (I think? maybe it was 55 still) night time limit. It's now 85 day/65 night MPH as of last summer when I last drove through.
But, the tl;dr is: when the posted limit was put in place, highway fatalities increased (quite dramatically from what I recall). No, I don't have any references to cite, it's just my recollection of events from nearly 30 years ago...