Diverging diamond interchange

Plan of rejected diverging diamond interchange in Findlay, Ohio

A diverging diamond interchange is a rare form of diamond interchange in which the two directions of traffic on the non-freeway road cross to the opposite side on both sides of the bridge at the freeway. It is unusual in that it requires traffic on the freeway overpass (or underpass) to briefly drive on the opposite side of the road from what they are accustomed.

Like the continuous flow intersection, the diverging diamond interchange allows for two-phase operation at all signalized intersections within the interchange. This is a significant improvement in safety, since no left turns must clear opposing traffic and all movements are discrete, with most controlled by traffic signals. Additionally, the design can improve the efficiency of an interchange, as the lost time for various phases in the cycle can be redistributed as green time; there are only two clearance intervals (the time for traffic signals to change from green to yellow to red) instead of the six or more found in other interchange designs. Some of the intersections in the design can be unsignalized. The left turn from the freeway off-ramp, for example, can form an auxiliary lane that then becomes an exit-only lane for the entrance ramp to the freeway in the opposite direction. Omitting the traffic signals for the left turn movements off the freeway only works well with single left turns and when short queues exist within the interchange on the arterial street.

History

Another diagram illustrating traffic movements on the side road

The only known diverging diamond interchanges are located in France in the communities of Versailles, Le Perreux-sur-Marne, and Seclin (see External Links, below). The interchange configuration was first introduced to America in a paper submitted to the 2nd Urban Street Symposium held in Anaheim, California in 2003 by Gilbert Chlewicki, a civil engineer from Baltimore, MD.

The interchange in Seclin (at 50°32′41″N 3°3′21″E) between the A1 and Route d'Avelin appears to have been made somewhat more specialized than in the diagram at right: eastbound traffic on Route d'Avelin intending to enter the A1 northbound must keep left and cross the northernmost bridge before turning left to proceed north onto A1; eastbound traffic continuing east on Route d'Avelin must select a single center lane, merge with A1 traffic that is exiting to proceed east, and cross a center bridge. All westbound traffic that is continuing west or turning south onto A1 uses the southernmost bridge.

Additional research was conducted by a partnership of the Virginia Polytechnic Institute and State University, the Turner-Fairbank Highway Research Center and a private consultant, and the results were published by Ohio Section of the Institute of Transportation Engineers.

A diverging diamond interchange is currently under construction in Utah County, Utah at the American Fork Main Street intersection with I-15, scheduled for completion in late 2010.

Advantages

• Provides for two phase signals with short cycle lengths, significantly reducing delay
• Reduced horizontal curvature reduces risk of off-road crashes
• Increases the capacity of turning movements to and from the ramps
• May potentially reduce the number of lanes on the crossroad, minimizing impacts to existing right-of-way
• Substantially reduces the number of conflict points theoretically improving safety
• Theoretically improves pedestrian safety.

Disadvantages

• Drivers may not be familiar with configuration, particularly with regards to merging maneuvers along the left side of the roadway or the reversed flow of traffic.
• Pedestrian would have to cross free-flowing traffic in freeway ramps. (Though this could be mitigated by signalizing all movements while not impacting the two-phase nature of the interchange's signals.)

Further Considerations

• No standards currently exist for this design and the design is extremely dependent on site-specific conditions.
• Additional signing, lighting, and pavement markings will be needed
• There is no accident history available in the North America Region, as no DDIs currently exist in North America.
• Local road should be a low speed facility (preferably under 45 mph posted speed on the crossroad approach, but this may be mitigated by utilizing a higher design speed for the crossing movements.)

Use in North America

While no complete diverging diamond interchanges currently exist in North America, several interchanges are being considered for conversion.

There is a sort of half-diverging-diamond interchange between Interstate 95 and Thurbers Ave. in Providence, RI ([1]). The extension of Thurbers Ave. to US Route 1 being a one-way prevents it from being a full diverging diamond, but the interchange directly with Route 1 keeps almost full functionality (except for the lack of an easy-access interchange from US 1 to I-95 South, though the area is full of I-95/US 1 interchanges elsewhere).

The Missouri Department of Transportation has plans to build four diverging diamond interchanges across the state: in Kansas City at the intersection of I-435 and Front Street, in St. Louis County at the intersection of I-270 and Dorsett Road, and in Springfield at the intersection of I-44 and MO-13 and at the intersection of U.S. 65 and Chestnut Expressway (Business Route 65).

This type of interchange is also being studied for possible use in Maryland along the Baltimore-Washington Parkway in Anne Arundel County, near the Baltimore-Washington International Thurgood Marshall Airport.

The Ontario Ministry of Transportation is also studying a diverging diamond as part of interchange upgrades at Highway 427 and Rathburn Road in Toronto.

In February 2007, the Oregon Department of Transportation accepted the DDI as the preferred alternative at Exit 24 (Fern Valley Road) along I-5 in Phoenix, in the southern area of Oregon.

In June 2007, Lexington, KY announced that it was also considering converting several New Circle Road interchanges to diverging diamond interchanges.

In 2008, NYSDOT announced that they were studying this alternative as a possibility for the I-590 / Winton Rd. interchange in Rochester, NY.

The Ohio Department of Transportation has also considered employing the interchange. ODOT planned to reconstruct the I-75/US-224/SR-15 interchange in Findlay to this configuration, but reconsidered. The agency is currently considering the configuration for the I-480/Tiedeman Road interchange in Brooklyn.

The Utah Department of Transportation is planning to use the diverging diamond to reconstruct the I-15 interchange at Main Street in American Fork.[2]

External links

• Animation of diverging diamond interchange at Dorsett and Interstate 270 in St. Louis.
• Web page for Springfield Missouri Diverging Diamond Interchange
• Web page for Missouri DDI at I-435 and Front st.
• 48.832115°N 2.152859°E / 48.832115; 2.152859 Map of a diverging diamond interchange in Versailles, France
• 48°49′49.1″N 2°29′35.3″E Map of a diverging diamond interchange in Le Perreux-sur-Marne, France
• 50°32′41.2″N 3°3′20.9″E Map of a diverging diamond interchange in Seclin, France