No links, but here are some references:
MANUAL ON UNIFORM TRAFFIC CONTROL DEVICES FOR STREET AND HIGHWAYS (US Gov. printing office),
TRAFFIC CONTROL DEVICES HANDBOOK,
MANUAL OF TRAFFIC ENGINEERING STUDIES(from Institute of Transportation Engineers),
TRAFFIC SIGNAL INSTALLATION AND MAINTENANCE MANUAL(from Institute of Transportation Engineers)
You might try to find a link for Eagle Signal or E.W. Bliss Company, in Iowa.
We purchased out traffic controls back in 1990 from Gades Sales in Littleton, Colorado. You might contact them for links.
Looking forward to you findings!