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Diverging diamond interchange

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Diverging diamond interchange

A diverging diamond interchange (DDI), also called a double crossover diamond interchange (DCD), 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. 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".

Contents

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 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.

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.

History

Prior to 2009 the only known diverging diamond interchanges were in France in the communities of Versailles, Le Perreux-sur-Marne and Seclin, all built in the 1970s. (The ramps of the first two have been reconfigured to accommodate ramps of other interchanges, but they continue to function as diverging diamond interchanges.)

Despite the fact that such interchanges already existed, the idea for the DDI was "reinvented" around 2000, inspired by the former "synchronized split-phasing" type freeway-to-freeway interchange between Interstate 95 and I-695 north of Baltimore.

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. By 2006, ODOT had reconsidered, instead adding lanes to the existing overpass.

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 37.2503°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. This interchange was a conversion of an existing standard diamond interchange, and used the existing bridge.

The interchange in Seclin (at 50°32′41″N 3°3′21″E) 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. The Federal Highway Administration released a publication titled "Alternative Intersections/Interchanges: Informational Report (AIIR)" with a chapter dedicated to this design.

Usage

As of January 2017, 90 DDIs were operational across the world including:

  • 3 in France
  • 1 in the United Arab Emirates
  • 86 in the United States of America

    • In 2015 plans were made to build Denmark's first DDI (in Danish, a Dynamisk Ruderanlæg) at the junction (exit 52) between E20 (the Funen Motorway; in Danish, Fynske Motorvej) and Secondary Route 168 (Assensvej), in Odense (at 55.361287°N 10.344932°E / 55.361287; 10.344932), replacing an existing diamond interchange. Construction of the interchange is scheduled to begin in the summer of 2016 and is expected to be complete by the end of 2017.

    Advantages

  • 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.
  • Potentially reduces the number of lanes on the crossroad, minimizing space consumption.
  • Reduces the number of conflict points, thus theoretically improving safety.
  • Increases the capacity of an existing overpass or underpass, by removing the need for turn lanes.

    • Disadvantages

  • Drivers may not be familiar with configuration, particularly with regards to merging maneuvers along the opposite side of the roadway or the crossover flow of traffic.
  • 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). 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 reenter 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:
  • 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, has received recent attention. 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. As of 2016, no such interchanges have been constructed.

    References

    Diverging diamond interchange Wikipedia
    (Text) CC BY-SA