Diverging diamond interchange

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Plan of rejected diverging diamond interchange in Findlay, Ohio
Plan of rejected[1] 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 that which 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.

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[edit] History

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[2].

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[3].

[edit] 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.

[edit] 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.
  • No standards currently exist for this design and the design is extremely dependent on site-specific conditions.
  • 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.)
  • 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.)

[edit] Use in North America

The first US diverging diamond interchange is currently being planned in Kansas City, Missouri. The Missouri Department of Transportation plans to use this design for the intersection of I-435 and Front Street in eastern Kansas City [1], and for the intersection of I-270 and Dorsett Road in St. Louis County.[4] 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 [5]. In February of 2007, the Oregon Department of Transportation accepted the DDI as the preferred alternative at Exit 24 along I-5 in Phoenix, in the southern area of Oregon.[6] [7] In June 2007, Lexington, KY announced that it was also considering converting several New Circle Road interchanges to diverging diamond interchanges.[8] In 2008 NYSDOTannounced that they were studying this alternative as a possibility for the I-590 / Winton Rd. interchange in Rochester, NY. [9]

[edit] References

  1. ^ a b http://www.landlinemag.com/todays_news/Daily/2006/Mar06/032706/033106-08.htm
  2. ^ New Interchange and Intersection Designs: The Synchronized Split-Phasing Intersection and the Diverging Diamond Interchange
  3. ^ Title of Paper (initial caps, boldface, on separate line, flush left)
  4. ^ I-270 and Dorsett Road Interchange Project. Missouri Department of Transportation. Retrieved on 2008-04-08.
  5. ^ TheStar.com - Page Not Found
  6. ^ Mail Tribune - At last, a solution - February 9, 2007
  7. ^ Southwestern Oregon Fern Valley Interchange
  8. ^ http://www.kentucky.com/454/story/106393.html
  9. ^ Unique Intersection Discussed in Brighton - 13WHAM.com

[edit] External links

Animation of diverging diamond interchange at Dorsett and Interstate 270 in St. Louis.

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