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From Roundabouts to Diverging Diamonds: Colorado's New Traffic Configurations

October 6, 2017

The Roundabout has always been one of my favorite design challenges.  I’ll always get excited when a new project comes along and it’s recognized that a roundabout is needed/desired and we get to dive into the intricacies of the design.  It’s effective, it’s beautiful, and it’s always a challenge to get it just right.  Roundabouts are very common in Europe and are now being implemented more and more often here in the United States.  You’ll even find a few double roundabouts in the area such as the Midland Ave. and I-70 interchange in West Glenwood Springs, and in Denver at the N. Pecos St. and I-70 interchange.   

However, as beautiful as a well-designed roundabout can be, there is a relatively new concept being implemented that is fascinating to me: the Diverging Diamond Interchange, or DDI.  The DDI allows traffic to cross over to the opposite side of the street as it moves through the intersection, giving vehicles a free left turn onto the highway on-ramp.  The number of stoplights through the intersection is reduced and the standard left turn movement at a light that typically requires significant storage space and long wait times is eliminated.  Studies have shown that the DDI can speed up the flow of heavily trafficked intersections and decrease the number and severity of traffic accidents.  Follow this link to see a very well done conceptual video of the DDI in action:  http://azc.cc/2uxDS7u  (Brieanna J. Frank, The Republic, azcentral.com) 

If you’d like to drive one yourself, head up to Highway 36 and McCaslin Blvd just south of Boulder.  It’s slightly alarming at first as the traffic pattern directs you to what is instinctively the wrong side of the road.  But, as you move smoothly through the intersection and see how easy and logical the left turn movement is, you’ll quickly see the benefit and begin to appreciate the thought that went into this concept.   

Check out the links below to learn more about this exciting new interchange concept, the DDI. 

In Colorado: 
http://www.cpr.org/news/story/roundabouts-diverging-diamonds-colorados-new-traffic-configurations

https://www.codot.gov/projects/US36ExpressLanes/project-overview/diverging-diamond-interchange

In Arizona: 
http://www.azcentral.com/story/news/local/phoenix-traffic/2017/07/19/adot-may-replace-roundabouts-diverging-diamond-design-interchange/490345001/

-Stephen Douglas, PE 

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