What is a DDI?
A Diverging Diamond Interchange (DDI), also known as a Double Crossover Diamond interchange, is a relatively new interchange design similar to traditional diamond interchanges.
What makes it unique is how left-turning and through traffic navigate intersections.
The DDI eliminates the requirement to wait for side road traffic looking to turn left onto the interstate on-ramp by removing left turns across oncoming traffic. While it may appear foreign and intimidating at first, it’s rather easy to navigate.
In a DDI, left turns across traffic are eliminated by transitioning traffic from the right side of the road to the left side of the road at each side road/ramp intersection. Once ramp intersections are cleared, traffic is brought back to the right side of the road. Therefore, drivers turning left to or from an interchange ramp are already on the left side of the road, meaning they no longer cross paths with oncoming traffic.
For example, when driving east to west on E. 1st Street in Ankeny, you’ll have the option to turn right onto the southbound I-35 entrance ramp or proceed through to a signalized intersection that safely moves you to the left side of the roadway.
In the event you travel through to the left side, you will be presented with another option of taking a free left turn onto the northbound I-35 entrance ramp or proceed through to a second traffic signalized intersection, which safely moves you back to the right side of the road once you’re through the footprint of the interchange.
Exiting the interstate is also safely accommodated with a DDI.
While on the exit ramp as you approach the side road intersection, which is E. 1st Street in this case, you’ll be required to either turn left or right onto E. 1st Street. You will not be required to cross traffic to turn onto E. 1st Street in either direction.
This allows for more efficient traffic movement and enhances the safety of intersections.