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Diverging Diamonds - High Efficiency at a Modest Price!

The DDI - an Award Winning Innovation!
What was one of Popular Science's "Top 100 Innovations" for 2009? What traffic innovation was featured in Time Magazine's Feb. 7, 2011 edition? Which project won AASHTO's 2010 Francis B. Francois Award for Innovation? The Diverging Diamond Interchange!!This site explains why freeway interchanges are often congested, and a bit more about how the DDI may prove to be among the lowest cost ways to do more with less!

Diagram of Movements in a Diverging Diamond Interchange

Why are interchange areas always so congested?

Freeway interchanges can be a nightmare. Even if you're not planning to get on the freeway, simply crossing it can take forever at peak periods. Part of the cause is that signals at each ramp on a traditional Diamond Interchange must have left-turn arrows, which introduces inefficiency.

SPUIs improved efficiency! a price.

The now popular Single-Point Urban Interchange, or SPUI, was a big operational improvement over the Diamond because it uses just one signal in the center, rather than two for each ramp. But the SPUI itself uses a four-phase signal with left-turn arrows, where the most efficient signals have just two phases. And because the signal must be in the center, ramps must approach the center diagonally, which requires larger bridge decks and wider spans - greatly increasing the cost of a SPUI relative to a Diamond.
Flows that are part of the Eastbound stream, vs. those that are part of the Westbound stream

DDI's: Safe, Efficient, Low Cost!

The Diverging Diamond is based on the idea that if you can eliminate the need for left-turn arrows, then signals will have maximum efficiency, fewer conflict points, and be able to serve more traffic with better safety and less congestion.

In this diagram of the USA's third DDI, recently opened in Utah, notice the orange stream where a free-right peels off to the south as at a normal diamond interchange.  Then the EB stream crosses over to the left side, making it possible to also make a "free left" to get on the freeway going North. Remaining traffic, just trying to cross the freeway, then crosses back to the normal side of road.  The free lefts can greatly reduce congestion, increase capacity and reduce conflict points. It is also easy to convert existing diamonds to DDI's, which means their benefits can often be achieved at a very low cost. Every situation is different, and DDIs will not be right everywhere, but could they be right for you?

One of the first three Diverging Diamonds, probably built in 1980's or 1990's - Paris, France

Where did this idea come from?

Three Diverging Diamonds have existed in France for over twenty years, with the first built in Versailles in the 1970's. The original inventors are unknown to us at this time. But in July 2003, Mr. Gilbert Chlewicki authored a paper titled "New Interchange and Intersection Designs: The Synchronized Split-Phasing Intersection and the Diverging Diamond Interchange", and presented it to the 2nd Urban Street Symposium in Anaheim, California. Dr. Joe Bared, a PhD and PE with FHWA, attended the presentation and was impressed enough to study the idea more and sponsor research. As Chlewicki and Bared's research became "discovered," a few state DOTs got serious about finding an opportunity to build one.

And the first is?...

After a few false starts, one that fell prey to fears over the safety of an unproven design, and another that was simply delayed by lack of funding, the first DDI outside of France was opened at I-44 and Hwy 13 on June 21, 2009  in Springfield, Missouri. The project cost just $3.2-million, because they were able to utilize the same bridge from the previous diamond interchange. MoDOT then opened the second DDI about a year later on July 12, 2010 at US-60 and National Avenue, also in Springfield. The Utah DOT opened the third on August 23, 2010 at I-15 and Main St. in American Fork. The 4th opened Oct 17, 2010 in St. Louis at I-270 and Dorsett Rd, and the 5th opened in Alcoa, Tennessee at US 129 and Bessemer St.  Both Utah and Missouri have at least one more currently under construction.

How well are they working?

Mr. Don Saiko, MoDOT Project Manager, reports that in survey's conducted after opening, 97% of respondents believe the projects made the area seem safer, 95% believe it is less congested, and 87% believe it is easier to drive than it was before. Mr. Saiko also notes that crash data collected in the first six months suggest a 50% overall reduction.

Fear that drivers would be confused and make mistakes in moving to the "wrong side of the road" have proven unfounded. Having driven this one in Utah, good channeling and signage make the transition very natural. You can hardly tell that anything unusual is happening. Hooray that DOTs across the country are finally taking this concept seriously!

First Diverging Diamond built in USA in Springfield, Missouri at I-44 & Hwy 13. Opened in June 2009. Converted an existing diamond at a cost of $3.2 million.

Where can I learn more?

DDIs and other Alternative Intersections and Interchanges can all be found at, where you can search for every Alternative Intersection that exists or has been planned anywhere in the world (or at least those that our members are aware of).  This site dedicated to DDIs, as well as, are sponsored by