Traffic signs
Traffic signs are like a programming language: they give instructions and set up constraints. Most importantly: there is more than one way to use traffic signs, but often a few of them would not be good. Some people are dumb, others can't read the language of the country they're in, and some are smart but bothered by any ambiguity. People who design roads, traffic signs, or deal with traffic, constantly have to think about all this. And they don't always do that.
I started caring about traffic signs (road signs, road markings, traffic lights, etcetera) when I first discovered that there are so many different shapes of arrows. I wanted to find out why, and gradually fell into the bottomless well of traffic engineering.
