## Thursday, March 26, 2020

### How Fast?

 Washington State Speed Limit Sign
As an engineer, I was trained to think in SI units.

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 "SystÃ¨me 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.