I don't remember if I ever posted this pic on this thread, but I recently posted this on another thread talking about freeways:
Confusing freeway route signs, 1960
gbcnet.com
In 1964 Caltrans decided to simplify the route numbers so that there wouldn't be redundant signage, which became more complicated when the Interstate System routing numbers came into use. Example, what is now I-5 from LA to San Diego used to be US-101. I think at one time, it was co-signed as both US-101 and I-5. Now, of course, the US-101 designation ends a little southeast of downtown LA.