Typically, the government passed legislation before technology was ready to meet the requirements. At first, engravers scrambled to do adhesive-backed letters applied to the face of signs along with adhesive-backed Braille strips. We call that process "Stick and Peel". Letters and Braille strips were glued on but easily removed. Engravers tried to route out the backgrounds along with Braille dots as a one-piece sign. This process was time-consuming and engravers offered little choice on colors. Etched metals appeared to be the onlyprocess acceptable for ADA use.

Today, photopolymer offers single piece construction and color choices along with metals. Polymer had its problems. A number of companies offering polymer did not invest in the processing equipment to assure the end-user that the Braille dots
would not be brittle and "flake" off.
Photopolymer offered relief graphics that most sign shops could make without environmental concerns. As this process caught on, the price of the photopolymer escalated. Raw material pricing today is not much less than zinc or magnesium metal. Raised polymer lettering is often hot-stamped with a thin layer of color to avoid silk-screening costs. Raised metal graphics are more often silk-screened with a hard enamel that is durable indoors and outdoors.
Zinc ADA signs offer very crisp definition in one piece of metal. It is a difficult metal to etch, more so than magnesium. Magnesium offers excellent definition as well but should not be left as a bare metal. It will oxidize to a dark color.
There were many alarmist that claimed that magnesium was a fire hazard. Usually the source was engravers trying to protect their market niche. Magnesium as a fine powder is quite flammable. Thicker metal plates are very difficult to ignite. Plastic signs will ignite quicker than a magnesium plate.
The other metals may be etched to ADA guidelines as well. For the more "enlightened" states such as California, Braille dots are supposed to be rounded.

Etching can produce this effect with a simple secondary etch process. Beside material strength and exterior application usage, etched metals can also offer logos and framed border in a single piece of material. For public use areas, the raised lettering is often left with its natural color which appears to be white. This foregoes tipped or colored letters that, when scratched, are difficult to repair. Despite the regulations, we still have designers specifying grained backgrounds with raised colored letters. This can be done with an etch cavity and lettering bonded into the cavity although disclaimers are usually supplied with product if a designer insists on a non-conforming format.