The AASHTO Green Book

Most state and local safety policies are based on the guidance recommended by the American Association of State Highway and Transportation Officials (AASHTO)—generally referred to by its popular name pronounced “ASH-tow”.

The AASHTO guidance for highway design, A Policy on Geometric Design of Highways and Streets, is known most popularly as the Green Book. The purpose of the Green Book is to recommend safe and efficient practices for the design of roadways. The recommendations contained in the Green Book are based on extensive research and study and generally provide a range of acceptable design criteria based on the type of roadway and the expected traffic volume for the facility. The FHWA has adopted the Green Book as the minimum standard for projects on the National Highway System (NHS), which includes the Interstate System and other selected principal routes and connectors to intermodal facilities (check with your state transportation department to see if your historic road is a part of the NHS). For all other projects, developed with Federal-aid funds or not, design is directed by the standards adopted by the state or local government. Almost every state and the majority of local governments have adopted the Green Book in whole or in part for use on their own projects. Remember the guidelines and recommendations contained in the Green Book do not become “standards” until adopted by your state or local government.

When working with the Green Book it is important to remember the flexibility contained in the range of criteria recommended. As noted, this flexibility is at the discretion of your state or local transportation department. It should also be noted, under special circumstances, that solutions outside the recommended range may be sanctioned by the FHWA for projects on the NHS. This special approval, known as a “design exception,” is based on a clearly articulated need and demonstration that the proposed solution will not lead to a safety problem.