Diverging Diamond Patterns: A Civil Engineering Persepective

Diverging Diamond Interchanges

Here is a little something for you Civil Engineers and Road “Architects”. Transportation and infrastructure in the modern world advances everyday. We see examples while driving, flying, and even biking around our cities, towns, and villages. Keeping traffic to a minimum for CE’s is as vital as CAD to an architect. Civil engineers attempt to fix this problem not only on roads and boulevards, but also interstates and their winding ramps.

As you may have seen on your own metropolitan area’s on and off ramps, there can be backup. Some days this is minimal, while others, it can be miserable. One method of transforming an interstate interchange to optimize vehicle speed efficiency is the new “Diverging Diamond Interchange”. Already being used in Missouri, this interchange accommodates left-turning movements at signalized, grade-separated interchanges of arterial and limited-access highways while eliminating the need for left-turn phasing. On the arterial, traffic crosses over to the left side of the roadway between the nodes of the interchange. Two-phase traffic signals are installed at the crossovers. Once on the left side of the arterial roadway, vehicles can turn left onto limited-access ramps without stopping and without conflicting with through traffic. The idea is simply revolutionary in the modern transportation world. A picture below explicates the system from a bird’s eye view. large_diverging-diamond

In the picture above, the grey parallel stripes represent the interstate, while the red and blue lines are the road with traffic signals. Most systems needed 4 traffic lights prior to the change, this plan only requires 2. The video below shows another aerial view, but this is a real interchange using this system. This is a better explanation for visual/ kinetic learners.

Advantages of the Diverging Diamond Interchange

  • 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, thus theoretically improving safety.
  • Theoretically improves pedestrian safety
  • Increases the capacity of an existing overpass or underpass, by removing the need for turn lanes.

 

Disadvantages of the Diverging Diamond Interchange

  • 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.
  • Pedestrians would be required to cross free-flowing traffic on freeway ramps. 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—unless the two signalized intersections are replaced with underpass/overpass structures (an expensive proposition and usually not possible within the existing right of way of the non-freeway road).
  • Exiting traffic cannot re-enter the freeway in the same direction without first leaving the interchange on the crossroad. This has several implications:
    • It makes it difficult to implement stops for express transit buses.
    • Drivers who accidentally take the wrong exit must turn around somewhere along the crossroad.
    • Emergency management cannot use the exit and entrance ramps to allow freeway traffic to bypass a crash at the bridge.
    • An oversize load can not use the exit and entrance ramps to bypass a bridge that is too low.
    • A diverging diamond can not be the parent interchange for a rebound interchange without using collector-distributor roads (local-express lanes).

Please leave comments and thoughts on what you think about the Diverging Diamond Interchange idea. I personally believe the pros outweigh the cons, it is easier to drive through than it looks. Trying to explain the system in words is almost impossible for most people to understand, so hopefully the picture and video did it for you. I want to hear feedback from not only Civil Engineers, but anyone who has driven in traffic before (99% of the USA).

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