Undoubtedly, no aspect of any kind of signage regulations is more exacting, or likely to be altered, than those governed by the Americans with Disabilities Act (ADA), which was originally enacted in 1990. Primarily, this concerned access for people with disabilities, such as wheelchair ramps, railings, handicapped parking, etc. But it also included stipulations for signage, specifically braille signage for people with limited or no vision. These regulations are outlined in ADA Accessibility Guidelines (ADAAG). Most of these regulations have been contentious, and they are changed often. The most recent iteration went into effect March 15, 2012. The following article outlines some of these specifics. http://www.nxtbook.com/nxtbooks/STMG/sott_201201/index.php#/68
Overall, on September 15, 2010, the U.S. Department of Justice published "2010 ADA Standards for Accessible Design," a 279-page document. http://www.ada.gov/regs2010/2010ADAStandards/2010ADAStandards.pdf. The most sign-specific section (703) begins on page 190. Here is a link to it. Note that the link is for all the 700 sections (700 - 708), but the sign-specific information is all in Section 703.