The Federal Highway Administration has ruled that the 2009 Edition of the Manual on Uniform Traffic Control Devices must be adopted by the states as their legal standard for traffic control devices within two years. Here is an excerpt of the Federal Register discussing the rule change.
The MUTCD contains all national design, application, and placement, standards, guidance, options, and support provisions for traffic control devices. The purpose of the MUTCD is to provide uniformity of these devices, which include signs, signals, and pavement markings, to promote highway safety and efficiency on the Nation’s streets and highways. The MUTCD is adopted by reference in accordance with Title 23, United States Code, Section 109(d) and Title 23, Code of Federal Regulations, Part 655.603, and is the national standard for all traffic control devices installed on any street, highway, or bicycle trail open to public travel.