In the United States traffic control signs installed in the right of way of streets have to conform to MUTCD (Manual on Uniform Traffic Control Devices) specifications. Cities and towns have some latitude on how certain kinds of directional signs can be designed and built, such as decorative directory signs in tourist-driven downtown areas. They also have latitude on what sign companies can do the work.
Regulations on things like stop signs, yield signs, etc. are very strict both in terms of designs and materials used. The same goes for any signs posted in the right of way on state or federal maintained highways. Those must follow MUTCD specifications. There are ever-changing regulations on retro-reflective standards. Signs must meet or surpass certain standards and maintain those standards in the field. When they get weathered enough the graphics or entire sign panel must be replaced. A lot of these signs use Type III or better "diamond grade" prismatic sheeting. The big green signs on freeways use the green prismatic vinyls from 3M or Arlon. Some of the smaller signs might use white diamond grade vinyl with translucent silk-screened graphics. There's a number of different methods being used, but they all must maintain reflectivity standards.
Recently the FHWA rescinded interim approval of the Clearview Highway type family for use on highway signs. States that have been using Clearview, such as Texas, have to revert back to the old FHWA Series Gothic type family when they replace worn out signs.
Normally highway signs are made by state Dept. of Transportation sign shops. Some cities even have their own traffic sign shops. Sometimes those agencies will sub-contract out the work to independent sign companies or even prisons. I've seen some strange, non-compliant things happen with some of these sub-contracted signs, such as improper typeface use (like Arial and Helvetica getting used), improper letter spacing or odd looking designs of Interstate highway shields. There's a few of those here in Oklahoma.
