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
, 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
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!