March 23, 2016 | ITRE

Traffic stopped at a diverging diamond interchange (DDI) in Cornelius, NC. ITRE researchers are developing a national guidebook for engineers and technicians on traffic signal design and operation at DDIs and adjacent intersections.

Traffic stopped at a diverging diamond interchange (DDI) in Cornelius, NC. ITRE researchers are developing a national guidebook for engineers and technicians on traffic signal design and operation at DDIs and adjacent intersections.

ITRE Highway Systems researchers are improving the national transportation guidance for practitioners who work with traffic signals at diverging diamond interchanges (DDIs). Institute staff, funded by the National Cooperative Highway Research Program (NCHRP), are leading the NCHRP 03-113 project which focuses on the operational analysis and traffic signal design at DDIs and adjacent intersections.

A DDI is a relatively recent interchange innovation in the United States for designing safer, more efficient intersections between freeways and arterial roads. ITRE has been studying the performance and effectiveness of DDIs since 2010. In August 2014, the Federal Highway Administration (FHWA) published earlier ITRE research findings in the second edition of its FHWA Diverging Diamond Interchange Information Guide.

The research team is nearing completion on a new guidebook for DDI signalization. The new guide should be useful to:

The NCHRP project focuses specifically on signalization in two areas of a DDI—the “influence” area and overall corridor improvements—where bottlenecks often occur. The new guidance will provide four basic phasing schemes along with variations based on specific project needs. Several levels of corridor strategies will be introduced along with the expected benefits based on volume and geometric combinations. Although the guidebook will provide a range of strategies to be considered, it is not meant to be prescriptive. Instead, the guidance will provide engineers and technicians with a set of tools and strategies to consider, along with the anticipated benefits based on each site-specific conditions.

Another phase of the project is slated to begin in spring of 2016 and will focus on updating geometric strategies and safety findings at DDIs for the third edition of the FHWA guide. These findings should be available in the summer of 2018. Please contact Chris Cunningham (cmcunnin@ncsu.edu) at ITRE for more information about this research and training offerings to support activities in your region.

The current edition of the FHWA Diverging Diamond Interchange Information Guide is available online at http://safety.fhwa.dot.gov/intersection/alter_design/pdf/fhwasa14067_ddi_infoguide.pdf.