Road signs

About road signs

The signs on U.S. roads follow the Manual on Uniform Traffic Control Devices, or MUTCD, published by the Federal Highway Administration. It sorts them into three classes. Regulatory signs give notice of traffic laws or regulations, warning signs give notice of a situation that might not be readily apparent, and guide signs show routes, destinations, distances and services.

The manual sets out what each sign looks like and where it goes, and state law can differ on what a sign requires of road users. The MUTCD itself notes one case: in most states, it says, the railroad crossbuck requires road users to yield to trains.

How U.S. signs are organised

The MUTCD standardises shapes and colours so that each class of sign can be recognised promptly. It reserves some shapes for single signs: the octagon for the stop sign, the downward-pointing triangle for the yield sign, the circle for the railroad advance warning sign, the pennant for the No Passing sign, the pentagon for school signs and the crossed boards for the crossbuck. The diamond is used for the warning series and the rectangle, square included, for the regulatory series.

Colour is the second code. Regulatory signs are mostly black, white and red, with red for stop, yield, do not enter and wrong way. Warning signs are black on yellow, and warnings about pedestrians and bicycles may be fluorescent yellow-green instead, as school signs are. Guide signs are white on green, road user service signs such as the hospital sign white on blue, and work zone signs black on orange.

How the MUTCD works

The first MUTCD was published in 1935, and it was the first edition to class signs as regulatory, warning or guide signs. The Highway Safety Act of 1966 gave the new U.S. Department of Transportation the authority to set national standards for traffic control devices, which made compliance with the manual mandatory across the country.

Each state has two years after a new edition to adopt it in one of three ways: the national manual as it stands, the national manual with a State Supplement, or a State MUTCD in substantial conformance with the national one. A state manual can be stricter, for instance by turning a recommendation into a requirement, but it cannot loosen a requirement of the national manual. The manual does not apply to parking areas or the driving aisles within them.

The current edition is the 11th, whose final rule was published on December 19, 2023. The Infrastructure Investment and Jobs Act, passed while it was being prepared, requires the manual to be updated every four years. The sign pages here quote the 2009 edition.

Differences by country or standard

Europe's signs follow a different tradition. The 1949 United Nations Protocol on Road Signs and Signals set out a system, based on European designs, that relies almost wholly on symbols without words. Writing in 1969, Jose M. Zuniga of the International Road Federation contrasted it with the system of the United States, Australia and New Zealand, which still used many written-word messages.

The biggest visible difference is the warning sign. The 1968 Vienna conference produced a draft convention that allows either a triangle with a red border on a white or yellow ground, the European form, or a yellow diamond with a black rim, the American one. Zuniga favoured the diamond: it holds a symbol 17 percent larger than a triangle of the same area, and England's Road Research Laboratory found its recognition range 10 percent longer.

The Worboys Committee concluded in 1961 that the UK should adopt the European system's main principles, and conversion of British signs began in 1965. The UK's Department for Transport sums up the result as circles give orders, triangles warn and rectangles inform, with a red ring or circle marking a prohibition. The STOP and GIVE WAY signs are exceptions to those shape rules, to give them greater prominence.

Markings, crossings and route markers

Pavement markings follow the same manual. Part 3 of the MUTCD uses yellow lines to separate traffic going in opposite directions and white lines to separate traffic going the same way. Whether a driver may cross a double yellow line to turn into a driveway is for state law: California’s and Oregon’s driver manuals allow it, each in its own words, and the double yellow line page quotes them.

The railroad crossing signs run from the crossbuck to the flashing lights and gates, the plaques and the pavement marking that go with them, and the blue Emergency Notification System sign, which the 2023 MUTCD requires on each approach to every highway-rail crossing. Among the route markers, exit numbers follow the mile markers, and the miles count from the south or west state line.