|Washington State Speed Limit Sign|
Is that Greek to you? Well, actually, it's French.
A "unit" is actually a unit of measure. Like a foot or a mile. "SI" is an abbreviation for the French term "Système Internationale" or "International System."
The basic SI units are the second (time), meter (length), ampere (electrical current), kelvin (temperature), candela (luminous intensity), mole (amount of a substance), and, kilogram (mass).
All other units are derived from these six. For example, speed (or velocity; and they are different) is measured in meters per second.
And there's the problem. When I see a speed limit sign without units, such as the one pictured here, I automatically think in SI units. So that should be 70 meters per second. Because that's how I was taught.
Now, my speedometer, for some strange reason, is calibrated in miles per hour. So I have to convert meters per second to miles per hour. And after some simple math (dividing, multiplying, making sure I keep my units straight), I find that 70 meters per second is around 156 miles per hour.
So is that the speed limit? Most cars can't go that fast (mine can). Would a cop buy that argument? Probably not.