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Diverging Diamond Interchange
A diagram illustrating traffic movements in the 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 crossover "X" sections can either be traffic-light intersections or one-side overpasses to travel above the opposite lanes without stopping, to allow nonstop traffic flow when relatively sparse traffic.
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. Its at-grade variant can be seen as a two-leg continuous flow intersection.
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.
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 2 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".
Pictures from the first diverging diamond interchange in the United States, in Springfield, Missouri Top left: Traffic enters the interchange along Missouri Route 13 Top right: Traffic crosses over to the left side of the road Bottom left: Traffic crosses over Interstate 44 Bottom right:Traffic crosses back over to the right side of the road.
Southbound approach to the I-44/Route 13 interchange in Springfield
Prior to 2009 the only 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. (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.
The interchange in Seclin (at 50°32?41?N3°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. The Federal Highway Administration released a publication titled "Alternative Intersections/Interchanges: Informational Report (AIIR)" with a chapter dedicated to this design.
As of November 29, 2018, 117 DDIs were operational across the world including:
The existing interchange between FM 60 (Raymond Stotzer Parkway) and FM 2818 (Harvey Mitchell Parkway) at Texas A&M University in College Station, Texas is being converted to a DDI; construction began on March 6, 2017 and is expected to last 25 months, with an estimated cost of $14.1 million.
The existing interchange between I-75 and State Route 725 in Miamisburg, Ohio, near Dayton, is planned to be converted to a DDI; the project will not be built until 2023 and its projected cost is $4.1 million, entirely funded by the state.
The existing interchange between I-70 and North Kipling Street (SH 391) in Wheat Ridge, Colorado, near Denver, is planned to be converted to a DDI. As of February 2019, neither the timeline nor the cost have been established.
The existing interchange at US 281 and Jones-Maltsberger Road in San Antonio, Texas will be converted to a DDI. This project has not yet been funded, so no timeline has been set.
Two-phase signals with short cycle lengths, significantly reducing delay.
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.
Increases the capacity of an existing overpass or underpass, by removing the need for turn lanes.
Costs significantly less than a normal interchange.
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.
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 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:
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.
3D computer generated DCMI
Double crossover merging interchange
DCMI traffic flow patterns
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.