A Guidance Report , January 2018 - FHWA-HRT-17-098

CHAPTER 3. RELATIONSHIP BETWEEN SPEED AND GEOMETRIC DESIGNGeometric roadway design practices in the United States rely on design controls and criteria set forth in the American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets, also known as the “Green Book.” The design speed is defined in the Green Book as “the selected speed used to determine the various geometric features of the roadway.” (AASHTO 2011) The Green Book either explicitly or implicitly uses the design speed concept to establish horizontal alignment, vertical alignment, and cross-section design elements. Examples include radius of curvature (R), stopping sight distance (SSD), braking distance (db), horizontal sight line offset (HSO), length of vertical curvature (L), maximum superelevation (emax), maximum side friction factor (fmax), and lane and shoulder widths.For the purposes of this guidance report, the designated design speed of a roadway is the s...