cars stop at the red light of a diverging diamond intersection

Diverging diamond interchange in Cornelius, NC.

February 11, 2021

ITRE researchers, in collaboration with engineering firms Kittelson and Associates and Kimley Horn and Associates, recently completed the Diverging Diamond Interchange Informational Guide, Second Edition for the National Cooperative Highway Research Program (NCHRP). This latest edition of the guide includes major updates on signal timing options, geometric considerations, and a nationwide safety evaluation.

The diverging diamond interchange (DDI) is a relatively new design to the United States for building safer, more efficient interchange facilities where freeways and arterial roads intersect. In many cases, DDIs can increase traffic throughput and safety without the need to widen bridge structures by using the existing infrastructure or building a parallel bridge. The research team led by ITRE developed much-needed guidance for transportation engineers on the geometric and traffic signal design of DDIs as well as safety and operational analysis of design alternatives. ITRE has been studying the performance and effectiveness of DDIs since 2010.

The research for the guide involved a literature review for this new design, interviews with experienced practitioners, identifying problem situations that occur at DDI installations for all modes of transportation, and computer simulation of promising design and control strategies. Recently, ITRE applied this new knowledge while conducting ten training workshops on signal timing options with transportation agencies representing a broad range of experience with DDIs.

“The updates provided in this second edition of the guide are extensive,” notes Chris Cunningham, ITRE program manager for Traffic Systems Innovation who served as principal investigator and lead co-author on the new guide. “This edition gives much more detail on specific design alternatives for the transportation practitioner to consider during every stage of the design and construction process for DDIs.”

The Diverging Diamond Interchange Informational Guide, Second Edition is now available online from the Transportation Research Board (TRB) which administers the NCHRP. Contact Chris Cunningham (cmcunnin@ncsu.edu) to learn more about this research and related training offerings.