+----------+
| /\_/\_/\ |
|(___A____)|
| )      ( |
|(   B    )|
| \__  __/ |
| C  \/    |
+----------+

The current design has no line between regions A and B. A and B are white and C is black; the only text is the number in region A-B. Shields on green guide signs (usually overhead) normally lack part C, and are simply a cutout with a number. New Jersey always uses region C, even on overheads. California always uses cutout shields with no region C; on standalone sign assemblies there is a small US above the number, and a black border around the outside.

In the early days (starting in 1926, when the US highways were established), Part C didn't exist. There was a black border around the outside, and a black line between A and B. Region A contained the state name. Region B contained the number, with a small US above.

Later shields lacked the state name in region A, and instead US was moved from B to A. In addition, non-cutout shields (having black in region C) started around this time.

In the 1950s, many states experimented with colored shields. Florida colored their routes by number until the 1990s. Kansas colored US 56 green. Arizona colored routes by direction. Mississippi, Washington, and maybe Connecticut tried colored routes in cities by direction. Rhode Island colored all sections red, with a white line between A-B and C, on US 1 and US 1-A in the 1970s. This replaced earlier signs (also used in Massachusetts) with all sections white and a black line between A-B and C. US 6 and US 44 may have also been colored in Rhode Island, maybe blue or yellow.

Source: http://www.us-highways.com/ussign.htm

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