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Superseded on 10/7/2015 by Relationship between Design Speed and Posted Speed

Subject: Design Speed Date: Aug 21, 1985
From: Chief, Highway Design Division
Washington, D.C. 20590
Refer To: HNG-21
To: Regional Federal Highway Administrators
Regions 1-10
Direct Federal Program Administrator

Recently there have been a number of questions regarding the selection of the appropriate design speed for Federal-aid highway projects. The purpose of this memorandum is to clear up any misunderstanding on this subject.

Mr. E. Dean Carlson's April 15 memorandum, "Implementation of New Design Criteria for Federal-aid Projects," sets forth the standards for new or reconstructed projects. The memorandum goes on to state that the selected design speed is to be equal to or exceed the posted or regulatory speed limit of the completed facility. The intent of this statement is to recognize that there is a relationship between design speed and the legal operating speed on a facility. In the design of any highway facility we should ensure that the selected design speed is logical with respect to the type of highway, the topography, the adjacent land use, and most importantly, that the design speed is consistent with the safe operation of the facility. Clearly, a posted speed (or the regulatory speed in the absence of a posted speed) implies definite driver expectations as to the safe speed for the highway.

The above statement should not be interpreted to mean that all roads must be posted with speed limit signs to match the design speed. In those situations where a highway cannot be designed in accordance with the speed limit, a design exception for the affected design element(s) should be processed in accordance with the provisions contained in 23 CFR 625(e). If only a portion of the highway does not meet the speed limit, as in the case of a sharp curve, advance curve signs and advisory speed plates should be used. In all cases, where the selected design speed is less than the posted or regulatory speed, mitigation should be considered.

/s/ original signed by
Ronald E. Heinz

Updated: 06/27/2017
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