Design

Superseded on May 5, 2016 by Revisions to the Controlling Criteria for Design and Documentation for Design Exceptions

U.S. Department of Transportation Federal Highway Administration

MEMORANDUM

Attached for your information are copies of the final rules for incorporating the latest AASHTO publication "A Policy on Geometric Design of Highways and Streets" (Green Book) and "A Policy on Design Standards- Interstate System" (I-Standards) into 23 CFR 625. The adoption of the new policies, effective June 1, 1993, will supersede previous editions of these two policies. For ongoing projects each division should discuss implementation with their respective State and reach an agreement on the application of the new policies.

For projects on the National Highway System (NHS), the criteria related to design speed, lane and shoulder widths, bridge widths, structural capacity, horizontal and vertical alinements, grades, stopping sight distance, cross slopes, superelevation, and horizontal and vertical clearances, contained in the functional Chapters VII and VIII of the Green Book, and in the I-Standards for the Interstate System, are the controlling criteria and require formal design exceptions when not met. In the absence of material covering controlling criteria in the functional chapters, material is to be sought in Chapters II, III, IV, IX, and X. You have previously been advised that the "clear zone" width, defined in the 1989 AASHTO Roadside Design Guide, is no longer considered a controlling criteria requiring a formal design exception.

In addition, for the Interstate System, the I-Standards include additional requirements that must be met such as a minimum of four lanes and a 20-year design period for new and reconstruction projects required by the law and full control of access.

Generally, the criteria in the Green Book functional chapters on local roads and streets and on collectors are not applicable to projects on the NHS. However, if highway segments functionally classified less than principal arterials are incorporated in the NHS by virtue of being STRAHNET Connectors or Intermodal Connectors, the standards used may be those appropriate for the functional classification of the segment taking into account the type of traffic using the segment.

On April 27 we distributed the AASHTO publication “Interim Selected Metric Values for Geometric Design (Interim Metric Guide)” The rulemaking process to incorporate this document into 23 CFR 625 is underway. The metric values in that document result in virtually the same dimensions as with the English system. For example, there will be no dramatic changes in standard lane or shoulder widths. Therefore, we do not expect the rulemaking to result in any substantive changes to the interim values. It is imperative, because of time constraints established by the metric conversion plan, that the activities to metricate State design standards and to actually design projects using metric measurements should be happening now. Therefore, the converted values in the AASHTO Interim Metric Guide may be used on Federal-aid projects at this time.

/s/
Thomas 0. Willett
2 Attachments