2011 No. 1 7

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DIVERGINGDIAMOND INTERCHANGE

A Shift in Traffic Flow for Safety and Efficiency

With the planned installation of a new BNSF intermodal facility along Interstate 35 in southern Johnson County, Kan., and steady residential and commercial growth in that area, traffic volumes are expected to increase dramatically. The Kansas Department of Transportation (KDOT) anticipates that in the next 30 years as many as 32,000 daily vehicle trips could pass through the area of Homestead Lane and I-35 as economic development, including the new intermodal facility, continues.

Not only will traffic increase in general, but the size of the vehicles will also increase. The intermodal facility and its ancillary buildings will serve as a hub for product distribution, meaning thousands of heavy trucks will be passing through the new interchange.

“There is a lot of regional growth accelerating the need for the project itself,” says Steve King, KDOT road design leader. “We have developed our plans for a new interchange with anticipation of the BNSF facility and other economic development so the traffic will be able to move efficiently.”

A French Revolution KDOT officials considered several traditional interchange options, including a simple diamond, a diamond with a loop ramp and a diamond with a flyover ramp. Traditional options likely wouldn’t be able to handle the traffic volume efficiently, and bridge

structures such as flyovers can increase the cost of the interchange by as much as $15 million.

“The type of interchange needed is driven by the pattern of traffic,” King says. “There will be a lot of traffic going to and from the intermodal facility and going north on I-35. It will have some really heavy left-turn traffic.”

Burns & McDonnell was hired to evaluate interchange options, provide environmental assessments and final design for the interchange. Steve Schmidt, Burns & McDonnell project manager for the project, was familiar with a new kind of interchange that would work well — a diverging diamond interchange (DDI) that makes left turns easier and travel safer.

Developed in France, the DDI is a relatively new concept to transportation departments in the United States. The interchange is a unique form of diamond interchange in which the two directions of traffic on the non-interstate roadway (Homestead Lane) cross to the opposite side of the road at each ramp intersection. Drivers approaching the first ramp intersection

can either turn right or proceed through by shifting to the opposite side of the road. At the next intersection, drivers are free to make an unopposed left turn, or they can continue through the intersection and cross back to the right side of the road.

Safe Changes Naturally, in a society where driving on the

right side of the road is the right thing to do, shifting the flow to the opposite side

is going to take some adjustments from the traveling public.

However KDOT officials and Burns & McDonnell are

confident the design of the ramp terminal

intersections, which will include guide

signs, pavement markings and

raised islands, will help guide

drivers through the intersection without

any problems.

“It is different and will take some adjustment from drivers,

but it is actually a much easier traffic flow,” Schmidt says.

Not only is the traffic flow better, but one of the most significant benefits this type

of interchange provides is improving safety.

“It is different and will

take some adjustment from

drivers, but it is actually a much easier traffic flow.”

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Two-phase traffic signal— freeflow left turn

HOMESTEADLANE

Traffic crosses overat intersection

I-35 Southwest Johnson County Interchange at Homestead Lane

The DDI allows for a two-phase traffic signal operation at the ramp intersections and significantly improves safety since left turn movements do not conflict with opposing through movements. The configuration allows seamless entry for traffic needing to enter the highway in both directions and only one encounter with a traffic signal for those vehicles making a left turn onto the highway.

“Because of the high volume of left-turn movements, this became a viable option,” Schmidt says. “Other options to reduce safety risks get pretty expensive pretty quick.”

The Missouri Department of Transportation completed the United States’ first DDI in Springfield, Mo., in 2009, and it has met with great success. The DDI planned for the area near the BNSF intermodal facility is expected to open in fall 2013 and will be the first in the state of Kansas.

“It is a new interchange configuration designed to improve traffic flow and safety,” says Joab Ortiz, senior public involvement specialist for Burns & McDonnell. “The use of DDIs is gaining momentum across the country.”

For more information, contact Steve Schmidt, 816-349-6816.

View a multimedia demonstration of traffic flow through the diverging diamond interchange at www.burnsmcd.com/ddi.

2011 No. 1