This.
The reason for having speed limits, in spirit, is that it's believed that driving faster than that will be deterimental to either yourself, or other drivers. The state of Montana
attempted to put just that into their laws, defining the speed limit as a "reasonable and prudent" speed. But that was found to be unenforceable, so now the speed limit is 75. But really, that letter of the law, of 75 mph, is merely there so the spirit is enforceable.
And in practice, other things are factored in to speed limits. Politics - speed traps being a prime example, but lower-than-expected limits in general often fall in to this. And the lowest common denominator - you might put the limit at 60 based on the middle of the night in the rain, but it may well be safe at 85 in the day when it's sunny out. Since you need a limit to be enforceable, the limit may be below what's "reasonable and prudent".
The circumstances would obviously vary in this case as to the details. Headlights, road conditions, a number of factors. I'm voting assuming that the conditions are favorable - good brights, low traffic, limited-access highway, little wildlife, etc. I think it's certainly possible that 80 mph is reasonable and prudent in various cases.
Within the U.S., Utah and Texas both have roads with 80 mph speed limits, and Texas has a road with an 85 mph speed limit. I don't know if these are all in the desert, but the point is that some authorities believe there area areas where this is safe as well. Not to mention the Autobahn, which has worked fairly well for many decades.
On a proper road, with sufficiently low traffic, I think it's also quite possible to drive safely at 75 mph at night, having done so in places such as Kansas, where it's perfectly legal. Maybe I just have really good night vision, but if it's 2 AM and you're in the middle of nowhere, it's quite easy to keep a track of everyone on the road. If anyone is approaching your blind spot, you've either seen them coming in the rearview mirror for miles, or they're on an on-ramp, at which point you should be expecting that possibility. Even an abandoned car should be visible well in advance such that it can be avoided, if the weather is not inclement.
Edit: Can we spoiler the large images plz?