May 24th, 2012

THE DDI ADVANTAGE

Optimize Mobility, by Catherine Higgins.

Presentations give insight on the design of Diverging Diamond Interchanges.

A group of engineers gathered for a lessons-learned discussion and tour of UDOT Diverging Diamond Interchanges. Participants listened to presentations given by Richard Miller, with Parsons Transportation Group lead designer of the Pioneer Crossing DDI, HG Kunzler with Lochner Engineering who designed the retrofit DDI at SR 201 and Bangerter Highway and  Michael Lasko with CH2M Hill who designed the SR 92 DDI. View each of the presentations here.

The event was held to coincide with a visit by Gilbert Chlewicki, a nationally known expert and proponent of the DDI. Though it’s “not a silver bullet” that will solve any traffic problem, Chlewicki believes the design is best used in an urban or suburban environment. In fact, Chlewicki predicts that the design should be considered first in an urban or suburban environment and will “make the SPUI extinct.”

UDOT has four DDIs, each built to meet mobility needs in their respective locations. According to UDOT’s Mark Taylor, Signal Systems Engineer, DDI’s have the following advantages:

DDI’s are more efficient because of fewer phases.  “Cars on the red have to wait for one phase instead of three.  Why, because left turns are eliminated.  Eliminating left turns mean that the green time that you would normally give to the left turn can be given to other movements.  Eliminating and shortening left turn green time is usually always more efficient for the overall intersection.”

DDI’s are safer.  “There are fewer conflict points for vehicles.  A conflict point is where conflicting movements would collide if every direction had a green.  A diamond interchange contains 30 conflict points.  A Single Point Urban Interchange (SPUI) has 24.  A DDI has only 18.  Fewer conflict points are safer.”

Glen Blackwelder with UDOT Region Three gave this presentation on DDI Operations:

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