Diverging Diamond Interchanges 

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.