AASHTO Green Book contains a general set of guidlines on road design, however, it is not meant to be a design manual. A great deal of flexibility is allowed, and designers are encouraged to tailor roads to particular situations.
What the Green Book Contains
The reference most often used by designers during the design of a highway project is commonly referred to as the Green Book. Its official title is A Policy on the Geometric Design of Highways and Streets. It has been published by the AASHTO, in one form or another, since the late 1930's, with the most recent edition issued in 1994. Although often viewed as dictating a set of national standards, this document is actually a series of guidelines on geometric design within which the designer has a range of flexibility. As stated in the forward to this document:
The intent of this policy is to provide guidance to the designer by referencing a recommended range of values for critical dimensions. Sufficient flexibility is permitted to encourage independent designs tailored to particular situations.
In order for the design criteria in the Green Book to become a standard, they must be adopted by a particular State (or may be set by court decision). The FHWY has adopted applicable parts of the Green Book as the national standard for roads on the NHS. These roads comprise all the interstates and some other primary routes. The design of roads other than those on the NHS is subject to the standards of the particular State. The standards adopted by a State are usually based on the Green Book criteria.