There is a lightly trafficked, mostly straight, 2 lane highway between where I live and the closest city large enough to need 5 digits on its population signs.

There is a lightly trafficked, mostly straight, 2 lane highway between where I live and the closest city large enough to need 5 digits on its population signs. The speed limit is 65. I drive 75 on it. Most people drive 85. In fair weather, with the right vehicle, 95 would be safe for most of it. A few years ago, the limit was 55, and at that time, a sheriffs deputy told me they don’t bother anybody going under 70. I don’t know what you really mean by acceptable, but there are plenty of roads with a 65 mph speed limit where 80 miles an hour is perfectly safe, and there are of course plenty of roads with a 65 mph speed limit where 70 mph is and unsafely fast. And there are road conditions in all of those roads where 45 mph would be on unsafe. It is difficult to make blanket statements about what a safe speed is knowing nothing other than the posted speed limit. To put the same point in a different context, remember that it wasn’t really that long ago that there was a national 55 mph speed limit, and many of the same interstates that used to have a 55 mph speed limit now have an 80 mph speed limit.