The colors I've traditionally used in creating highway sign graphics
are based on the Panose colors specified in the FHWA SHS text, as 
converted to RGB by Photoshop 7.

They are:

Brown  -- Panose 469 -- RGB 97,54,29
Green  -- Panose 342 -- RGB 0,110,85
Red    -- Panose 187 -- RGB 181,39,60
Blue   -- Panose 294 -- RGB 0,62,134
Yellow -- Panose 116 -- RGB 255,277,0
Orange -- Panose 152 -- RGB 230,113,0

The Nebraska DOT uses somewhat different colors in the sign specifications
they've placed on their website:

Brown  -- CMYK 38/63/93/36 -- RGB 119,78,36
Green  -- CMYK 100/0/79/9  -- RGB 0,155,103
Red    -- CMYK 0/100/65/0  -- RGB 237,23,36
Blue   -- CMYK 93/57/2/0   -- RGB 0,107,177
Yellow -- CMYK 0/0/100/0   -- RGB 255,242,0
Orange -- CMYK 0/51/87/0   -- RGB 247,146,51

...or at least that's what I get when I open the pdfs in Photoshop, and
use the color picker tool on them.

Personally, I like the Nebraska colors better, but they're inferred.
The FHWA are at least semiformal specifications.