After seven years of dedication, collaboration, and research, the Diverging Diamond Interchange Informational Guide, Second Edition is now available to transportation professionals everywhere! 

Diverging Diamond Interchanges (DDIs) help keep motorists safe, reduce bottlenecks and backups, and improve overall traffic flow to provide an efficient transportation route for drivers. As more DDIs were implemented throughout the United States, it became clear that the previous guidance was in need of an update. Traffic signal design and timing, evaluation of operational performance of DDIs currently in service, and implementing training were all important aspects of the research needed to create the new guidebook for America’s future with Diverging Diamond Interchanges. 

Research and Development: The DDI Guidelines 

The National Cooperative Highway Research Program (NCHRP) funded research and development for this new edition of the DDI guide. Kittelson engineers supported Institute for Traffic Research and Education (ITRE) staff at North Carolina State University over the course of seven years to create the finished product. Our goal was divided into three objectives: 

  • Research and develop guidance on the operational analysis and traffic signal design and operations of DDIs and adjacent intersections 
  • Identify, review, and evaluate the geometric design features and the associated safety and operational performance of in-service DDIs across the US 
  • Develop and deliver training and technology transfer program for the created guidelines 

The first phase of this research and development included looking to past installations of DDIs to understand the signal timing schemes being usedThis aided our understanding of best practices and lessons learned to consider for future design operations and optimization. From there, gathering insight and information from design, maintenance, and operation leaders informed our team not only of overall best practices, but also ways to develop new control strategies to improve challenging situations.  

A key consideration over the course of developing the guide was formulating general guidance that would apply to future DDIs across the board. Because interchanges are complex and full of individual moving parts, a generalized approach was necessary so that this type of interchange would be as successful as possible for all installations. 

We also knew the finished product needed to be accessible to various roles within the field of transportation. The final guide was created with four different roles in mind, including: 

  • Modelers and designers working to optimize the interchange’s design 
  • Signal designers applying operational strategies 
  • Signal technicians and timing engineers wiring traffic cabinets and programming controllers 
  • Operators who work with the interchange and adjacent intersections in real-time during the day 
Diverging Diamond Intersection

Diverging Diamond Interchange in Maryland.

Evaluating Design and Safety 

The next research goal focused on reviewing and evaluating the geometric design of the interchange, which largely included reviewing the design features and safety and operational performance of previously implemented DDIs in the United States. When this project began in 2014, only a handful of DDIs were operating within the United States. Now there are more than 100 DDIs operating throughout the county. In this phase, we determined the essential design characteristics of a DDI and how engineers should utilize those features in the design phase, and the safety and operational benefits of implementing a DDI. 

Guideline Unveiling and Future Training 

In the final phases of the project, our research team presented findings on the signal timing portion of the project via in-person training workshops. These one-day sessions were held over the spring and summer of 2019, reaching staff at 75 organizations, including ten state agenciesKittelson’s Bastian SchroederKevin Lee, Shannon Warchol, and Alison Tanaka led trainings across at eight locations across the United States, offering our methodologies and insights on signal timing at DDIs. 

What This Means for the Future of Diverging Diamond Interchanges 

“It’s important to highlight that while DDIs are the hot new thing, they won’t work at every interchange, every time,” Shannon notes. The new guidebook is a big step forward in understanding DDIs and making sure that when they are the best solution, they provide safety and efficiency for all users. 

“This is a guide that’s based more on experience in the real world of DDI’s and less on theory,” said Pete Jenior, who served as project manager over the second half of the project. “This is what the majority of DDIs in the US are going to be built from and designed off of. This is the definitive book now.” 

“This is what the majority of DDIs in the US are going to be built from and designed off of. This is the definitive book now.”

- Pete Jenior, Senior Engineer

We at Kittelson also had the privilege of leading the 2014 first edition of the DDI Guide, along with three other alternative intersection guidebooks on Restricted Crossing U-Turns (RCUTs), Median U-Turns (MUTs), and Displaced Left Turns (DLTs). In parallel research, we are looking forward to the release of NCHRP 07-25 on Pedestrian and Bicyclist Safety at Alternative and Other Intersections and Interchanges, which will be published as NCHRP Report 948 in the coming weeks. 

Diverging Diamond Interchange Informational Guide, Second Edition is now available on the Transportation Research Board website. Congratulations to all contributors for an amazing accomplishment that will serve travelers and transportation professionals throughout the country! For more information, or questions related to the projectfeel free to reach out to Shannon WarcholBastian Schroeder, or Pete Jenior.