What is the name of the U.S. manual that defines standard traffic signs and signals?

The U.S. manual that defines standard traffic signs and signals is the Manual on Uniform Traffic Control Devices.

Its commonly used abbreviation is MUTCD. The manual establishes national standards for traffic-control devices on public roads in the United States, including signs, traffic signals, pavement markings, and temporary work-zone controls. It is published by the Federal Highway Administration.

The purpose is uniformity: a driver should encounter broadly consistent meanings and designs across different states and local jurisdictions. The manual includes specifications for colors, shapes, symbols, placement, size, and permitted wording.

The MUTCD is not a general driving-law code. States and local authorities enforce their own traffic laws, and some jurisdictions may adopt supplements or additional provisions within the national framework. The manual’s role is to standardize the devices that communicate those rules to road users.