Diverging diamond interchange
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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
Apart from an existing Diverging Diamond in Versailles, France (see External Links, below), there are currently no other such interchanges anywhere in the world. 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[1].
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[2].
[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 [3]. 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 planning a diverging diamond as part of interchange upgrades at Highway 427 and Rathburn Road in Toronto [4]. 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.[5] [6]
[edit] References
- ^ http://www.urbanstreet.info/2nd_sym_proceedings/Volume%202/Chlewicki.pdf
- ^ http://www.ohioite.org/DDI-Paper-Veh-Ped-Oper.pdf
- ^ http://www.landlinemag.com/todays_news/Daily/2006/Mar06/032706/033106-08.htm
- ^ http://www.thestar.com/NASApp/cs/ContentServer?pagename=thestar/Layout/Article_Type1&call_pageid=971358637177&c=Article&cid=1148468892002
- ^ http://www.mailtribune.com/archive/2007/0209/local/stories/fernvalley-diamond0.htm
- ^ http://www.oregon.gov/ODOT/HWY/REGION3/fvi_index.shtml
[edit] External links
- Map of a diverging diamond interchange in Versailles, France
| Types of road junctions | |
| Interchanges (grade separated) |
Cloverleaf - Diamond - Directional T - Diverging diamond Parclo - Trumpet - SPUI - Stack - Three-level diamond - Raindrop |
| Intersections (at-grade) |
Box junction - Continuous flow - Hook turn - Jughandle - Michigan left Quadrant roadway - Roundabout - Superstreet - 3-way junction - Traffic circle - Bowtie |
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