The new interchange at I-580 and Moana Lane in Reno brought about a round of “firsts” for the Nevada Department of Transportation (NDOT).
The construction of a Diverging Diamond Interchange (DDI), a design that is growing in popularity in the U.S., has set the stage for future interchange projects that aspire to be built in an innovative and efficient manner.
With just 64 working days to complete the project, prime contractor Granite Construction renovated the old interchange into one that accelerates traffic and reduces the risk of accidents.
The $6.9 million project began in Sept. 12, 2012, when the interchange was partially shut down for reconstruction. Work proceeded in two phases and was complete before the Thanksgiving holiday on Nov. 21, 2012.
Project improvements include: drive time savings, efficiency, congestion relief, safety, cost and time savings, reduction in head-on crashes and reduction in maintenance.
This is the first transportation project in northern Nevada to use the Construction Manager at Risk (CMAR) method of procurement.
CMAR, which is also frequently used in the construction of schools, is “particularly suited to renovation type projects because this type of project is time sensitive, has a complex scope of work, can be subject to changes that become apparent late in the design process and requires a high level of complex construction management.” according to the Washoe County School District.
A final NDOT first on this project is the inclusion of “green” bike lanes resulting in safe bicyclist access.
Phase one included reconstructing the I-580 southbound Moana Lane off-ramp and prep work to reconstruct the northbound on and off-ramps.
Phase two included reconstruction of the I-580 Moana Lane northbound on and off ramps, the southbound on ramp, and Moana Lane from just west of the southbound off-ramp to just east of the northbound off-ramp.
The Diverging Diamond
Diverging diamond interchanges are often used at intersections located beneath or above freeway bridges.
“NDOT wanted to improve this interchange without a lot of land acquisition or extended disruption to traffic,” said NDOT spokesperson Scott Magruder.
Diverging diamond interchanges have been in use for years in France. The first diverging diamond interchange in the United States was completed in 2009 in Springfield, Miss., followed by other successful diverging diamond interchanges in states such as Utah. Plans for diverging diamond interchanges also are proceeding in many additional states.
Diverging diamond interchanges were ranked one of Popular Science’s top 25 “Best of What’s New.”
How It Works
At a diverging diamond interchange, traffic briefly crosses over to the left (opposite) side of the roadway, safely guided by traffic signals at each crossover. This allows vehicles to turn left onto freeway on-ramps without stopping and without conflicting with through traffic.
In other words, traffic is briefly routed to the opposite side of the road at an intersection.
Because vehicles are now traveling on the left side of the road, they are on the same side of the road as freeway on-ramps and are able to take the freeway on-ramp or other exit without waiting in separate turn lanes as in traditional intersections.
Through traffic simply travels through the intersection and switches back to the right side of the road after the intersection, all guided by roadway signs and markings.
Diverging diamond interchanges feature signs, striping, curbs, headlight glare screens and concrete barriers to guide and safely separate opposing traffic as it moves through the interchange.
The Benefits
The free, unsignalized left turns onto freeway on-ramps, etc. helps reduce congestion.
“The key to the DDI is the ’free’ left turn,” said Magruder.
There are fewer signal phases and a shorter wait at traffic lights by eliminating signalized left-hand turn lights.
DDIs often reduce by half the number of conflict points at which vehicles can potentially collide, and make “wrong way” entry onto the freeway ramps extremely difficult.
Additionally, they can be more cost and time-effective to build, as they often require less land and fewer potentially-costly changes to existing road structures.
DDIs also provide heightened pedestrian/bicyclist safety with the construction of designated, enclosed shared use paths built in the middle of the diamond interchange.
Granite Construction, headquartered in Watsonville, Calif., has built two other Diverging Diamond Interchanges and works on 2,000 construction projects annually.
The I-580 project called for 20,000 cu. yds. (15,292 cu m) of roadway excavation, and installation of 9 manholes and 17 drainage inlets. Crews layed 9,000 ton (8,163 t) of aggregate base and 3,800 ton (3,446.6 t) of dense grade paving.
They also poured approximately 6,500 cu. yds. (4,970 cu m) of PCCP and set more than 15,000 linear ft. of white paving forms. In 64 working days on this interchange, Granite logged over 25,000 man hours with zero injuries.
For more information, visit www.Moana395.com.
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