Ask yourself: how many people drive 40 miles, taking an hour to get there? Or 20?
Maybe on the freeway, you might go 60 miles in an hour, but it’s very likely that you’ll be going 55 part way, then 65, then 60.
Unless, of course, you have cruise control, in which case none of this matters to you.
Besides that, there’s the matter of the ridiculous unit: miles. A mile is 5280 feet, or 1720 yards, A yard is 3 feet, and a foot is 12 inches. The inch was originally defined as 1/12 of a foot (though where the “12” came from is anybody’s guess. The inch is nowadays divided into 1/2, 1/4, 1/8. 1/16, and so on). Some say the word “inch” is related to the word for thumb: in Sanskrit, for example, “angulam” = “inch’; “anguli” = “finger”.
“Mile” comes from the Latin “mille”, or “thousand”, because the average Roman soldier’s pace was 5 feet, and at that time, a mile was 5000 feet. Since then, inflation has taken effect.
In the interests of improving the world. I present a much more meaningful way of describing automobile speed: meters per second (m/S).
We’re much more likely to drive a meter at a constant speed. (I resist the temptation to do something about our awkward system of time units: seconds, minutes, hours, days, weeks, months, and years. The French tried, during the Revolution, but that failed miserably.)
During the brief time before everybody adopts this unquestionably superior system, it will be useful to have a conversion factor.
To convert miles per hour into meters per second, multiply by 0.44704. To go the other way, multiply by 2.237. If you use 0.45 and 2.2, you’ll only be off by less than a percent or so.
The test of a good system of units is whether or not we can represent common quantities in a manageable number of digits. This is why we switch from pounds to tons once the measurement of pounds goes past 4 digits.
So then, the average highway speed of 65 mi/hr is 29 m/S. Quite manageable. 20 mi/hr = 9 m/S. (Actually 8.94/S, but certainly close enough for road work.)
Since speed limit signs don’t include units (points off, there), there will naturally be a few cases of people driving 65 m/S, or 145 mi/hr, but, as this is perfectly understandable, little more than a warning and information booklet need be issued.
(There was a case, some years back, where a car driven by foreign tourists was stopped for going about 100 mi/hr on the freeway. When the Highway Patrol finally pulled them over, the driver explained that the sign clearly read “101”.)