- March 31, 2021
- Perspectives
Highlights of the Diverging Diamond Interchange (DDI) Informational Guide, Second Edition
Stacie Phillips, PE
Transportation Engineer
The Diverging Diamond Interchange (DDI) Informational Guide, Second Edition is now available on TRB’s website. The 200-page manual is seven years in the making and the result of contributions from transportation experts from across the nation including Kimley-Horn’s Stacie Phillips, PE, who played a key role in developing Guidance for Traffic Signals at Diverging Diamond Interchanges and Adjacent Intersections (NCHRP 3-113)—the research that led to the DDI guide. Here are some key highlights from that research.
Key Takeaways
- Many aspects of the design, such as pedestrian and bicycle paths and the location of signal equipment, need to be considered earlier than they would in a more traditional interchange.
- Both traffic signals operate with only two vehicular phases, offering a terrific opportunity for the use of low cycle lengths, which is useful for clearing traffic out of the center of the interchange regularly and for minimizing wait times for pedestrians, bicyclists, and vehicles.
- While the crossing path is not intuitive for pedestrians, with proper consideration, a DDI can be beneficial for pedestrians due to low signal cycle lengths, short crossing distances, and low vehicular speeds.
- Signal progression requires extensive fine tuning due to the short distance between crossovers; low, but variable speeds within the interchange; and extensive interplay between adjacent intersections, freeway ramps, and crossover movements.
“DDIs became popular so quickly that there were already a lot of them constructed before we knew what worked well. This guide will provide a lot of help to practitioners in utilizing lessons learned from that first generation of DDIs.”
Learn More
Order your physical copy of Diverging Diamond Interchange Informational Guide, Second Edition or download the free PDF from TRB’s website.
About the Expert
Stacie Phillips, PE
Stacie has more than 17 years of experience as a transportation engineer, specializing in intersection design, operations, and safety. Having designed over 1,000 traffic signals in North Carolina and coordinated more than 2,000 traffic signals nationally, she is thoroughly familiar with balancing operational and safety objectives of all modes of transportation. She serves on the Transportation Research Board Traffic Signal Systems Committee and was the 2018 President of the North Carolina Section of the Institute of Transportation Engineers (NCSITE). Stacie is a national expert on traffic signal design and timing and teaches about these topics for the State of North Carolina.