I was just wondering how useful they really are. Thus my question - what are they really for?
They are there for: To prevent the combination of carelessness, inattention, or stupidity, or mental lapse of judgment
If your city expected you to know the speed limits all over town, by memory only, by reading a previously published bulletin, isn't there some chance that you might forget if you were distracted ?
However if you are going down a street, and suddenly see a big bold posted metal sign, with big bold letters and numerals, right in front of you: "SPEED LIMIT 15 MPH" ... That should refresh your memory.
Without that sign being visibly present, you might errantly speed right on through a 15 mph school zone, running down 15 children crossing the road, or T-Bone a school bus at 65 mph
In Japan, the conductor and engineer, both call out all the signs and signals, out load to each other, and signal each other by a hand salute, with white gloves on, that they heard and understood each other, and comprehended each and every spoken word, and that they both saw each and every sign and signal correctly.
They do not just rely on memory ... They read and acknowledge to each other, each and every sign and signal, by reading it, and speaking it out loud, acknowledging each other, there is almost zero chance of mistake.
Without signs, there is a 50-50 chance, or even greater, that an engineer might blow right on through a restricted speed zone ... Hoping and praying that someone has total 100% recall, of a written bulletin. is like hoping and praying that the person who packed your parachute was not a complete idiot, and errantly filled your parachute backpack with Boy Scouts of America pots and pans, knives, forks and spoons ... and when you pull the ripcord camping stuff flies out, and it goes: "chinkity klink clank ding ding ..."