Selection of Design Speed Values

A revised definition for design speed has been adopted by AASHTO and included in the 2001 Green Book. Although this revised definition addresses several concerns, it does not include guidance on how to select the most appropriate design speed value. A summary of AASHTO policy, state practices, and international practices with regard to design speed selection demonstrates substantial variation in the processes used to select design speed. An analysis performed using operating speed as the measure of effectiveness validates some of the AASHTO criteria used to select design speed. However, the analysis also indicates that terrain (one of the AASHTO criteria used to select design speed) may not be related to operating speed, and its inclusion in the design speed selection process is justified in roadway cost considerations. Four possible solutions are presented for selecting the most appropriate design speed value for a better relationship between design speed and operating speed.