Diverging Diamond Interchanges
What is a Diverging Diamond Interchange?
The diverging diamond interchange (DDI) is an alternative interchange design that is becoming more widely used across the Commonwealth and other States. The primary feature of a DDI is that the opposing traffic streams on one of the crossing roads is reversed for a short section through the interchange area. The crossing of the two opposing traffic streams is accomplished at two traffic signal controlled intersections on either side of the interchange. For certain combinations of traffic volumes, the DDI offers benefits compared to a conventional diamond interchange because the DDI simplifies the signal phasing and progression of vehicles through the interchange.
When should a DDI be considered?
Designers consider DDIs when a particular location has the following traffic characteristics:
- Heavy volumes of left turns to and from freeway ramps
- Moderate but unbalanced crossroad traffic volumes through the interchange
- Left turn-related safety concerns at the intersections within the interchange
- Need for additional capacity without widening the roadway and bridges
Why do anything at Nutley Street?
The existing I-66 and Nutley Street interchange is a full cloverleaf interchange. As shown in the image below, the existing interchange provides short distances for weaving between the adjacent loop ramps on Nutley Street and on the collector-distributor (C-D) lanes on I-66.
Existing Weave Areas on Nutley Street and I-66
(Source: Google Earth)
This limited weaving distance increases the potential for crashes, as indicated by the yellow and red colors in the images below. Based on these crash densities, segments of I-66 in both directions within the Nutley Street interchange area were identified as crash hot spots. Furthermore, crash rates at the Nutley Street interchange are higher than statewide averages for interstates and are among the highest crash rates within the I-66 project area.
Existing Westbound/Eastbound Crash Density at I-66/Nutley Street
(Source: Final Transportation Technical Report, August 2016)
With the existing full cloverleaf interchange, pedestrians walking along Nutley Street on either side of the existing bridge over I-66 have to cross at the four loop ramps. Also, more land would be needed to “push out” these loop ramps and create a bigger footprint for a cloverleaf interchange that would accommodate the express lanes.
Three different design concepts were previously considered. Each of these design concepts for the I-66/Nutley Street interchange eliminated weave movements on I-66, by replacing some, or all, of the existing loop ramps. The DDI concept was selected based on reduced construction cost compared to the other two alternatives and improvements to operations on I-66 and Nutley Street. While the DDI introduces new traffic signals on Nutley Street, the design will provide efficient traffic flow for future traffic growth while also providing safer operations on I-66. In addition, the new traffic signals will allow bicycles and pedestrians to safely cross Nutley Street and the ramps to and from I-66 without conflict from vehicles traveling on free flowing loop ramps.
How will the Nutley Street DDI work?
Traffic coming from eastbound I-66 can get onto Nutley Street as shown in the image below:
Traffic from Vienna or Lee Highway that wants to get onto eastbound I-66 can do so as shown below:
Traffic coming from westbound I-66 can get onto Nutley Street as shown in the image below:
Traffic from Vienna or Lee Highway that wants to get onto westbound I-66 can do so as shown below:
What are the Advantages and Disadvantages of a DDI?
A DDI reduces congestion by allowing traffic to keep moving through its intersections, reducing the number of traffic signal phases, and giving more green time to the through movements. It also improves safety by eliminating left turns against oncoming traffic, providing better sight distance at turns, and reducing the opportunities to conflict with other vehicles, which results in fewer crashes.
While providing safety and operational benefits, DDI’s come with disadvantages such as the increased potential for wrong-way movements, pedestrian confusion as to which way traffic is coming from, and information overload for the driver due to the increased level of signage needed for the DDI. However, these disadvantages can be mitigated through proper design and driver education.
A comprehensive list of advantages and disadvantages of a DDI compiled by FHWA, can be found at the following location:
Where are DDIs being used in Virginia and across the U.S.?
As of August 2017, there are currently three DDIs in operation and two that are in the design stage across Virginia. The locations of these DDIs are shown in the map below.
Map of Operational/In Design DDI in Virginia (as of August 2017)
Simulation videos of the three DDI in operation in the Commonwealth can be found at the following links:
- US 15/I-66 DDI in Haymarket: https://www.youtube.com/watch?v=hJZt1kFshkQ
- I-581/Valley View Boulevard DDI in Roanoke: https://www.youtube.com/watch?v=WLQ1KroVUY4
- I-64/US 15 DDI in Zions Crossroads: https://www.youtube.com/watch?v=kYOCC1oUivo
More than half of the 50 states have DDIs in operation or in the planning or design stages.
Map of Operational & Planned DDI Locations in the United States
(Source: Diverging Diamond Interchange Informational Guide, FHWA, August 2014, https://safety.fhwa.dot.gov/intersection/alter_design/pdf/fhwasa14067_ddi_infoguide.pdf)
An informative video from FHWA that highlights DDI operations and benefits can be found here: