Three thoughts on NO PARKING signs

These aren't thoughts about whether or not to park; they're really just random thoughts about the design of the NO PARKING signs themselves.

One, color. 
I'm sure I learned this back-in-the-day when I was taking my driver's test, but for some reason I just noticed again the color scheme. Black lettering is information; red is prohibition. So a NO PARKING sign has a black P with a red circle and a red tilted slash through it, the same way the black-on-white ONE WAY sign sits above the white-on-red DO NOT ENTER sign.

Two, font.
I look at the aesthetic of traffic signs differently ever since I watched the documentary "Helvetica". As much as I know how weird it is to see my peers Geeking Out on nerdy math facts, it was a lot of fun to watch these graphic designers Geeking Out on fonts. In the documentary, there are some people who get teary-eyed about Helvetica helping to reunify Europe in the decade that followed the second world war; there were others who could barely contain their rage over the use of Helvetica as a tool of the corporate capitalist oppression of creativity and individuality. I seldom think of fonts as moral choices (oh, except that once I read that "Century Gothic" uses less ink than other kinds of fonts), . . . but I do now realize that the font of each NO PARKING sign comes with historic, sociological, economic, and political significance.  Go figure.

Three, the symbol itself.
The NO PARKING symbol has an interesting relationship to the math symbol plus (+).  It was in the 1500s that a mathematician and physician named Robert Recorde invented arithmetical symbols that you'd think would have been around for forever: he was the first to use the symbols =, +, —. 

He invented the now-familiar equals sign to replace the phrase "is equal to"; he invoked geometry and used two parallel line segments of the same length, because after all what could be more equal than those? 

For the other arithmetic symbols, he turned to shipping for inspiration. People used standard sizes of crates to box things up, with a number written on it to indicate how much it held. If the box wasn't all the way full, as was common, you'd write a long line and then write the missing amount. So "25 —— 3" would mean that the crate that could hold 25 pounds, but the crate was 3 pounds light. 

Occasionally, however, the crate would be overfull, In which case you'd put a small slanted slash through the long line (it's hard to type it, but it would kind of look like this: "25 —/— 3").  So the plus symbol really comes from meaning "not subtracted from" in the same way that the NO PARKING symbol means "parking, NOT!": that little slash through them has a common symbolic ancestry.  Neat, huh?!