What is a Diverging Diamond Interchange (DDI)?
A Diverging Diamond Interchange is:
- An innovative interchange design that improves safety and mobility.
- Similar to a conventional diamond interchange, which is the most common in the United States, where the exit/entrance ramps make a diamond shape between the freeway and surface street. Right-hand turns are handled the same at both a DDI and a diamond interchange.
- Different in how it handles left turns, since:
- The intersections with the freeway ramps smoothly move traffic on the surface street from the right side of the roadway to the left side of the roadway as the road crosses the freeway.
- Traffic is on the left-hand side of the roadway between the signals at ramp intersections, all left turns occur at the entrance/exit ramps without having to cross opposing traffic.
- Road geometry, signs, and pavement markings working together to make driving through the DDI very simple.
Federal Highway Administration (FHWA) on Alternative Intersections: Diverging Diamond Interchange video (7:39)
Flow of traffic in existing Big Beaver Road configuration
Flow of traffic in a typical Diamond Interchange configuration
Flow of traffic in a Diverging Diamond Interchange configuration
Graphic: Proposed Big Beaver Interchange crossovers, turns, and ramp merges.
Where are DDIs going in?
Two are approved to go in at the I-75/Big Beaver Road and I-75/14 Mile Road Interchanges.
- An existing DDI design is located on I-75 at University Drive in Auburn Hills. The difference is in the location of the interchange; at University Drive, the DDI is located above I-75, and the new DDI’s will have road located below the freeway.
Proposed I-75 at Big Beaver Interchange video (1:44 no audio)
Why do we need DDI’s as part of this project?
DDI’s help facilitate safer movement for vehicles, pedestrians, and cyclists while increasing throughput.
- Reduces conflict points between vehicles and non-motorists by nearly 50 percent.
- Eliminates many of the most severe crashes, which happen at right-angles.
- Increases visibility for drivers turning.
- Reduces potential of people driving the wrong way on entry ramps.
- Separates non-motorized users (bikes and pedestrians) by routing them through the median or along the sides of the roadway.
- Simplifies crosswalks and involves crossing fewer lanes at a time.
- Provides a simple two-phase signal design with shorter cycle lengths (total time for the traffic signal to give a green light for ramp traffic and a green light for the surface street traffic).
- Allows simple left and right turns from all directions.
- Increases the number of left-turning vehicles without the need for additional lanes.
- There is more space between the signalized intersections at the ramps and one less signal.
- Reduces the construction cost compared to other interchange forms.
- Requires fewer lanes therefore, existing bridges and right-of-way can be utilized.
- Utilizes a smaller project footprint, which means fewer impacts to adjacent areas.
How do you navigate a DDI and what to expect?
In Cities where DDIs have been built, drivers say they do not even notice the crossover intersections (shifting to the left side of the road). This can be attributed to the road geometry, signs, and pavement markings that make navigating a DDI easy for the driver. Below are the four travel paths made within a DDI: Through, left turns onto I-75, left turns from I-75, and right turns to/from I-75.