Do you even know what the MUTCD is? How do you know they are "nonsense rules?"
Think of all the highway signs, signals, pavement markings and other roadside devices you encounter during the course of your travel over a week, a month or longer. The MUTCD is a set of standards for every one of them ... and its purpose is to standardize layouts, appearances and color schemes based on decades of meticulous research.
Most (if not all) states -- which have the legal authority to develop their own standards for roadways that aren't part of the National Highway System -- adopt the MUTCD standards as their STATE standards for highway signs.
Here's just one sample page from a prior edition of the MUTCD. Note that each of the individual signs and pavement markings in this schematic plan has its own set of standards for size, color and letter/number font.
