Diverging diamond interchange

A diverging diamond interchange (DDI), also called a double crossover diamond interchange (DCD),^[1][2] is a type 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 is customary for the jurisdiction.^[2] The crossover "X" sections can be traffic-light intersections or one-side overpasses to travel above the opposite lanes without stopping, the latter of which allows nonstop traffic flow when traffic is relatively sparse.

Like the continuous flow intersection, the diverging diamond interchange allows for two-phase operation at all signalized intersections within the interchange. That is a significant improvement in safety since no long turns (e.g. left turns where traffic drives on the right side of the road) must clear opposing traffic and all movements are discrete, with most controlled by traffic signals.^[3] Its at-grade variant can be seen as a two-leg continuous flow intersection.^[4]

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. That is because 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.

A diverging diamond can be constructed for limited cost at an existing straight-line bridge by building crisscross intersections outside the bridge ramps to switch traffic lanes before entering the bridge. The switchover lanes, each with two side ramps, introduce a new risk of drivers turning onto an empty, wrong-way, do-not-enter, exit lane and driving the wrong way down a freeway exit ramp to confront high-speed, oncoming traffic. Studies have analyzed various roadsigns to reduce similar driver errors.

Diverging diamond roads have been used in France since the 1970s. However, the diverging diamond interchange was listed by Popular Science magazine as one of the best innovations in 2009 (engineering category) in "Best of What's New 2009".^[5]

The design also is promoted as part of the Federal Highway Administration's Every Day Counts initiative which started in 2011.^[6]

History[]

The first known diverging diamond interchanges were in France in the communities of Versailles (A13 at D182), Le Perreux-sur-Marne (A4 at N486) and Seclin (A1 at D549), all built in the 1970s.^[7] (The ramps of the first two have since been reconfigured to accommodate ramps of other interchanges, but they continue to function as diverging diamond interchanges.)

In the United States in 2005, the Ohio Department of Transportation (ODOT) considered reconfiguring the existing interchange on Interstate 75 at U.S. Route 224 and State Route 15 west of Findlay as a diverging diamond interchange to improve traffic flow. Had it been constructed, it would have been the first DDI in the United States.^[8] By 2006, ODOT had reconsidered, instead adding lanes to the existing overpass.^[9][10]

The Missouri Department of Transportation was the first US agency to construct one, in Springfield at the junction between I-44 and Missouri Route 13 (at

37°15′01″N 93°18′39″W / 37.2503°N 93.3107°W / 37.2503; -93.3107 (Springfield, Missouri diverging diamond interchange)). Construction began the week of January 12, 2009, and the interchange opened on June 21, 2009.^[11][12] This interchange was a conversion of an existing standard diamond interchange, and used the existing bridge.^[13]

In November 2016, the Delaware Department of Transportation completed the conversion of the DE-1 and DE-72 (Wrangle Hill Rd) overpass west of Delaware City, DE into a diverging diamond interchange.^[14] The interchange is viewable via live traffic camera on the DelDOT website.^[15]

The first diverging diamond interchange in Australia opened to traffic by the Queensland Department of Transport and Main Roads on November 28, 2019. The interchange is located in Caloundra, Queensland and connects Caloundra Road to the Bruce Highway. The former interchange was upgraded and converted as part of the larger Bruce Highway Upgrade Program.^[16][17]

In December 2019, the Virginia Department of Transportation completed the conversion of the interchange at Courthouse Road and I-95 in Stafford, VA into a diverging diamond interchange.^[18]

In 2020, the California Department of Transportation (Caltrans) completed the first diverging diamond interchange in California. An interchange at State Route 120 and Union Road in Manteca, California was converted to this interchange and opened to traffic on November 25th.^[19][20]

The first interchange in Canada opened on August 13, 2017, at Macleod Trail and 162 Avenue South in Calgary, Alberta.^[21] Followed by one in Regina, Saskatchewan the next year as part of the Regina Bypass project.^[22]

The interchange in Seclin (at

50°32′41″N 3°3′21″E / 50.54472°N 3.05583°E / 50.54472; 3.05583) between the A1 and Route d'Avelin was 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 and the Turner-Fairbank Highway Research Center and published by Ohio Section of the Institute of Transportation Engineers.^[23] The Federal Highway Administration released a publication titled "Alternative Intersections/Interchanges: Informational Report (AIIR)"^[24] with a chapter dedicated to this design.

Use[]

Operational[]

As of 16 November 2020, 135 DDIs were operational across the world including:

• 2 in Canada
• 1 in Denmark opened September 17, 2017^[25]
• 2 in France, built in 1970s
• 1 in Malaysia
• 2 in Saudi Arabia
• 2 in South Africa
• 1 in the United Arab Emirates
• 1 in Australia with another under construction
• 123 in the United States of America with 29 more under construction^[26]

Advantages[]

• Two-phase signals with short cycle lengths, significantly reducing delay.^[27]
• Reduced horizontal curvature reduces the risk of off-road crashes.
• Increases the capacity of turning movements to and from the ramps.
• Potentially reduces the number of lanes on the crossroad, minimizing space consumption.
• Reduces the number of conflict points, thus theoretically improving safety.^[2]
• Increases the capacity of an existing overpass or underpass, by removing the need for turn lanes.
• Costs significantly less than a normal interchange.^[28]

Disadvantages[]

• Drivers may not be familiar with configuration, particularly with regard to merging maneuvers along the opposite side of the roadway or the crossover flow of traffic.^[29]
• Pedestrian (and other sidewalk-user) access requires at least four crosswalks (two to cross the two signalized lane crossover intersections, while two more cross the local road at each end of the interchange).^[30][29] This could be mitigated by signalizing all movements, without impacting the two-phase nature of the interchange’s signals.
• Free-flowing traffic in both directions on the non-freeway road is impossible, as the signals cannot be green at both intersections for both directions simultaneously.
• Highway bus stops are appropriately sited outside the interchange.
• Allowing exiting traffic to re-enter the through road in the same direction requires leaving the interchange on the local road and turning around, e.g., via a median U-turn crossover. This affects several use cases:^[29]
  • Drivers who take the wrong exit
  • Bypassing a crash at the bridge
  • Allowing an oversize load to bypass a low bridge

Further considerations[]

• No standards currently exist for this design
• The design depends on site-specific conditions.
• Additional signage, lighting, and pavement markings are needed beyond the levels for a standard diamond interchange.
• Local road should be a low-speed facility, preferably under 45 mph (72 km/h) posted speed on the crossroad approach. However, this may be mitigated by utilizing a higher design speed for the crossing movements.

Double crossover merging interchange[]

A free-flowing interchange variant, patented in 2015,^[31] has received recent attention.^[32][33][34] Called the double crossover merging interchange (DCMI), it includes elements from the diverging diamond interchange, the tight diamond interchange, and the stack interchange. It eliminates the disadvantages of weaving and of merging into the outside lane from which the standard DDI variation suffers. A highway U-turn requires weaving, however. As of 2016, no such interchanges have been constructed.

See also[]

• List of road interchanges in the United States
• Single-point urban interchange (SPUI)
• Jughandle
• Hook turn
• Michigan left
• Superstreet

References[]

Further reading[]

External links[]

Wikimedia Commons has media related to Diverging diamond interchanges.

• DDI Guideline – A UDOT Guide to Diverging Diamond Interchanges, Utah Department of Transportation, June 2014
• Transportation engineers discuss the design of the fifth U.S. DDI in Alcoa, TN
• Video of Paramics Traffic Simualtion software modeling a Double Crossover Diamond Interchange
• Diverging Diamond Interchange Visualization, Instructional video on how to drive in a DDI, NCDOTcommunications, published on 10 March 2011
• Images of Diverging Diamond Interchange in Springfield, Missouri the first in North America.
• Animation of Diverging Diamond Interchange under construction at Elmhurst Road on Interstate 90 near Elk Grove, Illinois, to be completed in 2016
• Animation of Diverging Diamond Interchange at I-590/Winton Road (Rochester, New York)

Examples[]

• 48°49′56″N 2°09′10″E / 48.832115°N 2.152859°E / 48.832115; 2.152859 Diverging diamond in Versailles, France
• 48°49′49.1″N 2°29′35.3″E / 48.830306°N 2.493139°E / 48.830306; 2.493139 Diverging diamond in Le Perreux-sur-Marne, France
• 50°32′41.2″N 3°3′20.9″E / 50.544778°N 3.055806°E / 50.544778; 3.055806 Diverging diamond in Seclin, France
• 37°15′N 93°19′W / 37.25°N 93.31°W / 37.25; -93.31 Diverging diamond in Springfield, Missouri