Really, you can blame the whole "UX" fad for destroying sensible HMI/HCI based design.
The stop sign is a classic case of form following function. Bold red colour, so you notice it. Unique shape, so you can tell what it is before you get close enough to read it. Simple and to the point, designed by engineers.
UX brings in a shit load of bollocks around it rather than making it as simple as it needs to be.
Exactly this. UX as a whole is a cancer on modern computing -- nothing more than a combination of follow-the-leader and a circle-jerk. All it takes is for someone presents a (completely wrong) idea and, as long as they are authoritative about it, the other UX sheep will view that opinion as gospel, not to be questioned but only blindly followed. This might be a teacher at a school or a company like Google.
A perfect recent example is this Stack Exchange question regarding traffic signals. An ignorant (but inquisitive) person asks why traffic signals are always three vertical lights instead of some cool new UX-y system of LEDs and poor contrast. An answer posted which sounded very authoritative (but included no references) and had a few pretty pictures was immediately up-voted by the other UX sheep, even though the answer is completely wrong. The author eventually went and made some edits to claim his view was "just historical" to cover up the fact that he was glaringly wrong about the issue of color blindness.
You can see this behavior everywhere. Microsoft following Apple, Mozilla following Google. It has nothing to do with something being empiraclly or evidently better -- it's simply everyone following the hipster cool kid in class around because, well, he wouldn't be popular if he wasn't right!
We've had computer usability studies for decades now which have provided some keen insights into how people intuit the function of computer (some very interesting ones from the original Mac and Windows 95 timeframes). UX, however, has nothing to do with research or study -- it's little more than populist bullshit.