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Diverging Diamond Interchanges 

Traffic driving through a Diverging Diamond Interchange in PennDOT District 12

A Diverging Diamond Interchange (DDI) is an intersection and interchange design that enhances and simplifies the operation of diamond-style interchanges by moving the crossroad traffic to the opposite side of the roadway at the on- and off-ramps to the interchange. Intersection and Interchange Geometrics is a Federal Highway Administration (FHWA) Every Day Counts Round 2 (EDC-2) innovation that Pennsylvania championed. 

How Does It Work? 

Use of a DDI eliminates left turns across oncoming traffic. At the interchange, crossroad traffic in both directions is transitioned or crossed over to the left side at a signal-controlled point and then transitioned back to the right side at a second signal-controlled point after passing through the interchange. At the second point, left turns can be made without crossing oncoming traffic. The road configurations, signage, and pavement markings assist drivers to easily move through the intersection.   

DDIs can be considered for locations that have: 

  • Heavy left-turn volumes at on- and off-ramps 
  • Moderate, but unbalanced crossroad traffic volumes through an interchange  
  • Left-turn-related safety concerns at an interchange  
  • Need for additional capacity without the ability to widen the roadway or bridge 
 


What Are The Benefits? 

The DDI is a design innovation that improves safety and mobility by reducing the number of conflict points with oncoming traffic. DDIs provide a timely, cost-effective design solution, and can often be constructed at a much lower cost compared to conventional interchange designs. The design takes up less space, reducing the impact of surrounding property, and provides for improved capacity, now and in the future, by requiring fewer lanes to handle the same amount of traffic than a conventional design. 

DID YOU KNOW... Compared to a conventional diamond interchange, the DDI reduces vehicle-to-vehicle conflict points by nearly 50 percent and eliminates many of the most severe crash types.

Innovation in Motion  

PennDOT District 12 opened the first DDI in Pennsylvania in September 2016 at the Interstate 70/U.S. Route 19 interchange in Washington County as part of the Interstate 70/Interstate 79 Widening Project. District 12 is also designing a second DDI at the Interstate 70/State Route 51 interchange in Westmoreland County. District 12 created webpages for the Interstate 70/Interstate 79 Widening Project and Interstate 70/State Route 51 projects, which include a video and materials related to DDIs. These materials were also displayed at public meetings.  

Traffic driving through a Diverging Diamond Interchange in PennDOT District 12

Involving the public and obtaining their support is very important to the success of this type of interchange design. District 12 found the key to gaining public buy-in to be communication and engaged stakeholders through public meetings, private clubs and organizations, conferences, and news media. They also created brochures for both DDI and roundabouts and offered driving simulations on their website. The driving simulations were very successful, as it helped people understand the DDI concept and showed how to easily maneuver through the intersection. 

A DDI is also in development in PennDOT District 8 at the Shrewsbury interchange of Interstate 83 in York County, and another is planned for the U.S. Route 222/U.S. Route 322 interchange in Lancaster County

More Information

For more information about this innovation, contact the STIC Management Team.