This link has been poster a number of times in other threads. It is uswful for those who whic to accurately depict Highways and roads in the era they model.
https://ceprofs.civil.tamu.edu/ghawkins/MUTCD-History.htm
For example you will find in the 1948 MUTCD that the standard color for a STOP sign in 47 States is yellow. The exception is California where it is red.
In the 1954 Revision to the 1948 edition the specified color is changed to red (no more yellow).
Note however that there were many exceptions to the latest MUTCD standard. - Roads to older standards and jurisdictions do not always follow the standards.
In California at the intersection on a very rural unpaved mountain State highway east of Paso Robles there was a very worn yellow STOP sign in 1959 or 60.
Also in California there were State route signs and guide signs with the American Automobile Association (AAA) logo. This site has some photos. There were many still around in the 1970's. The last were placed in 1969.
http://caltrafficsigns.com/pictures/thumbnails.php?album=10
http://caltrafficsigns.com/pictures/thumbnails.php?album=11
I believe the AAA also installed requlatory signs. My fuzzy memory tells me that the yelow STOP sign I mentioned above had the AAA logo.
Timeline of AAA signs
http://www.caltrafficsigns.com/chronology.php
California Sign Specifications
http://www.dot.ca.gov/trafficops/tcd/specs.html
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
How very interesting!! And useful, for model railroaders. Who have roads.
Thank you for posting this.
Ed
At least one yellow stop sign survived in rural Massachusetts near Warren until the 21st century.
Disclaimer: This post may contain humor, sarcasm, and/or flatulence.
Michael Mornard
Bringing the North Woods to South Dakota!
My N scale railroad is modeling the CP in eastern Ontario. I have an old copy of the Ontario road map that has samples of highway signs on the back. I'll be reducing and modifying scanned images to apply to the railroad. I don't know if the current map still has the road signs.
Anyway, that might be a source for road signs elsewhere. A province's tourism ministry might be a source. Often they will send you paper maps for free.
Genesee Terminal, freelanced HO in Upstate NY ...hosting Loon Bay Transit Authority, run through Amtrak and CSX Intermodal
CP/D&H, N scale, somewhere on the Canadian Shield
Some resources for Canada
Wikipedia article has some examples https://en.wikipedia.org/wiki/Road_signs_in_Canada
Drivers Handbook http://www.mto.gov.on.ca/english/dandv/driver/handbook/section3.1.0.shtml
British Columbia Sign and Pavement Marking manual
http://www.th.gov.bc.ca/publications/eng_publications/electrical/most_pm.pdf
Traffic Control Manual (Construction) Newfoundland Labador
http://www.tw.gov.nl.ca/publications/TrafficControlManual.pdf