Texas highway would be the first to allow 85 mph speed limit

[A HERETIC I AM]

60. Then let me put it this way;

That 75% of the traffic is flowing within a blanket, if you will, of just a few miles per hour variant of that target. It is my experience driving on freeways (or motorways, if that makes you more comfortable(!) that the vast majority of drivers do travel within a 4 or 5 MPH average and that range makes up the 75th percentile. If the speed limit is 70 and the conditions are fine, 75% of the cars on the roadway will be within just a few MPH of that speed generally. I drove up I 95 today from Melbourne, FL to Jacksonville (speed limit - 70 MPH almost the whole way), a trip of about 185 miles. The drive took me through varying bands of rain, some of it torrential. The vast majority of traffic seemed to flow within a few MPH of each other and that varied by as much as 30 MPH depending on how hard the rain was coming down. The point I am trying to make is that the drivers who pose the greatest risk were those that were either driving way above that 4 or 5 MPH blanket average or way below it.

I understand the point you make;

99% of people drive "outside that 75th percentile", by definition. You can measure a speed which one quarter is driving faster than, and three quarters slower than.

But I think you are trying to be too specific. Yes, of course 75% of drivers will not cover the same amount of ground at the exact same rate. But for the most part, it is certainly true that of those drivers that made the same trip I did, I arrived at my destination within just a few minutes of 75% of them.

I am not "try(ing) to make it a rule to drive at that speed", rather I am positing that it is much safer to drive at the average of the larger group, give or take 4 or 5 mph, not 10 or 20 or 30 MPH.

Is that perhaps a bit better?

As far as your use of the word in quotes 'undertaken', I had actually included that as a reference in my post re: my experience driving in England, but elected to edit it out. It's nice to know there are readers on DU who get that point.