Most regulatory signs are produced in bulk, using methods a lot different than your typical custom sign.
In stead of vinyl plotters and finished blank panels, your typical regulatory sign starts off with raw materials that are cut with sheers or dies. From there, the substrate can be painted, screenprinted or covered with reflective material using methods that are not found in a typical retail sign shop. Relfective materials can be applied using heat, vaccuum, or pressure and the message can be screenprinted or die cut vinyl.
While the manufacturing process is relatively simple, the cost of the equipment to do this efficiently is quite expensive.
One of the educational channels, like Discovery or TLC just ran a program showing how these signs are made. Unfortuantely a quick search didn't turn up the program, but I want to say it was How it's Made, or something like that.
Checkers