A diverging diamond interchange (DDI) is a relatively new concept where motorists cross-over from driving on the right side of the road to the left side of the road as they go across an interchange. The crossing road then switches back to standard right side driving on the other side of the interchange. The benefit of this crossing is to make all turns like a standard right turn, close to the curb and without other conflicting traffic.
The first DDI was built in the United States in Springfield, Missouri in 2009. Since then, they’ve been built in at least 17 states, with many more being planned, designed, or in construction. DDI’s are rapidly gaining in popularity because they improve safety by reducing conflict points (14 at the ramps compared to 26 at the standard diamond interchange ramps) and by reducing cost with a skinnier bridge deck (multiple turn lanes are not needed). In some cases, particularly if there is a heavy left turn volumes, the capacity of the DDI intersections are much greater than other types of interchange configurations.
Theoretically, a DDI is safer than other types of interchanges due to the reduced conflict points because fewer conflict points should result in fewer opportunities for crashes. Initial data suggests that DDIs are working as expected with fewer crashes and better operations. A performance evaluation of the Interstate 44 and Route 13 DDI in Missouri found a 46 percent reduction in all crashes and a 72 percent reduction in left turn type crashes. The public also perceived the DDI as being safer and operating better than a standard diamond interchange. While the initial studies are good, more research is needed to confirm they are a safety enhancement.
Since DDIs are a new concept, engineers are still working out a few issues in the operations and best practices for the signing and striping. The most optimal timing of the traffic signals is also being debated, with choices between operating with one or two controllers, interconnecting or allowing free-flow operations, and whether to signalize or use yield control for the left and right turns. Ensuring that drivers understand the movement of traffic and knowing where to look for conflicting traffic is also being discussed and evaluated, particularly for driving on the “wrong-side” of the road.
Benefits of Diverging Diamond Interchanges
- Reduced conflict points compared to other types of interchanges.
- Higher capacity for turning traffic, particularly left turns, compared to other types of interchanges.
- Slower speeds and ‘calmer’ traffic due to the curved design of the DDI.
- Decreased delays and vehicle stacking since the two-phase operation of the traffic signals can have shorter cycle lengths.
- Easy U-turn operation for the freeway driver to turn around back to a missed exit.
- Potentially better for on the road bicyclists because of lower vehicle speeds.
- Reduced construction costs through re-use of an existing bridge or smaller bridge deck compared to other types of interchanges.
Limitations of Diverging Diamond Interchanges
- Less efficient for through traffic due to the controlled cross-over of traffic rather than a free-flow movement.
- Confusion to new drivers in the area due to driving on the ‘wrong’ side of the road.
- Less efficient with low volumes or low demand.
- Eliminates the through movement for ramp traffic, meaning a freeway driver cannot exit and re-enter the same direction of freeway if they take the wrong exit.
- More difficult to time the traffic signals to get desired progression across the interchange.
- Potentially less pedestrian friendly due to the multiple crossings of free-flowing turning movements and the potential for traffic to approach from an unfamiliar direction.
- Potentially less transit friendly as stops need to be positioned outside of the intersections.
- More right-of-way required on either end of the bridge to allow for the crossing movements.
Design Guidelines for Diverging Diamond Interchanges
Currently, there are no official standards for this interchange type. Design criteria are heavily based on site specific situations and the experiences of each agency (lessons learned from previous projects). Based on growing experience with operations at DDIs, the following guidelines are generally recommended:
- Crossover (crossing) angles between 40-50 degrees to minimize the likelihood of wrong way driving, aid traffic calming and safely accommodate larger trucks with their higher center of gravity.
- Tangent length approaching and following the crossover between 10 and 15 feet to allow for a ‘straight’ crossing through the intersections, similar to how roundabouts are designed.
- Curve radii approaching and following the crossover between 150 and 300 feet to accommodate a safe speed of travel.
- Lane widths a minimum of 12 feet, but desired 14 or 15 feet to better accommodate the turning traffic (checking truck turning movements for overlap is a key task).
- Intersection design speed of 20 to 30 mph, with ramp turning movements at 15 mph.
- Sight distance principles should be followed according the AASHTO Green Book.
Resources Related to Diverging Diamond Interchanges
- Missouri’s Experience with a Diverging Diamond Interchange, http://library.modot.mo.gov/RDT/reports/UnNumbrd/or10021rpt.pdf
- Driver’s Evaluation of the Diverging Diamond Interchange, http://www.fhwa.dot.gov/publications/research/safety/07048/
- Double Crossover Diamond Interchange, http://www.fhwa.dot.gov/publications/research/safety/09054/
- Diverging Diamond Interchange Performance Evaluation, http://ntl.bts.gov/lib/36000/36000/36038/or11012.pdf
- Website of the Gilbert Chlewicki, the researcher who brought the DDI concept to the US, http://www.divergingdiamond.com/benefits.html