Construction of a “diverging diamond” interchange (DDI) on the Catawba Avenue bridge over I-77 will take only about three months and shouldn’t be a major disruption for drivers or the environment once it begins in 2013, N.C. Department of Transportation officials said Tuesday. But the same cannot be said for the nearby “bookend” intersections at U.S. 21 and Torrence Chapel Road, which are likely to remain as-is for a while despite town officials’ hope to up-fit them as part of the same project.
“The DDI is a template for what we want to implement in other projects,” NCDOT Traffic Engineer Jim Dunlop said. “It’s a simple fix to a pretty significant traffic problem.”
Mr. Dunlop and other DOT officials were at Cornelius Town Hall Tuesday for a public workshop on the DDI. Citizens asked questions and saw design plans for the project, set to begin in July 2013.
Funding for the project comes from a $6 million federal earmark the town received in 2004 for design and engineering of a better intersection at I-77 and Catawba Avenue. Last March, town officials approached DOT officials and asked them to consider a DDI, which would require less work than the alternative – a single point intersection – and would cost about $30 million less. Cornelius would become only the second town in the country with such an interchange, after Springfield, Mo., which has two.
DOT officials liked the idea so much they since have planned four other DDIs in the area: two in Cabarrus County, off I-85 at the intersections with Poplar Tent and NC-73, and one at the intersection of Mallard Creek Road and I-485.
CONSTRUCTION WITHOUT MAJOR DISRUPTION
From an engineering and construction standpoint, Mr. Dunlop said, the Cornelius DDI is ideal: The road footprint remains almost the same, with only slight changes at the on/off ramps, the bridge will not need to be widened and the road will not need to be shut down during the constructions process, though at times traffic may be funneled into one lane. For the DDIs in Springfield, Mo., workers only shut down the intersections over night before opening them for the new traffic patterns the next day.
“The current pattern can basically be kept as-is until they’re ready to make the switch,” Mr. Dunlop said.
A DDI allows for better traffic flow because of its unique design – the two directions of traffic on the non-freeway road cross to the opposite side on both sides of the bridge at the freeway, preventing them from having to turn left across multiple lanes of oncoming traffic and allowing cars entering and exiting the freeway to do so more smoothly. The ramp design can make drivers’ wait time five times shorter, according to studies, Mr. Dunlop said. The DDI plan for Cornelius also includes a protected lane about 8-feet wide between the two driving lanes that will have a short wall barrier and will allow safe passage through the intersection for pedestrians and cyclists.
BOOKENDS WILL HAVE TO WAIT
DOT officials estimate the project will cost only around $3.5 million. The town wanted to use $2.5 million in left over funds for work at the adjacent intersections, so they can better handle traffic flowing from the DDI. Officials with the Federal Highway Administration must approve how the funds are spent, and after months of discussion it seems unlikely that they will approve spending money on the “bookends,” Town Project Manager Tyler Beardsley said. So far there is no official ruling in the FHA decision, but DOT officials echoed Mr. Beardsley’s prediction.
But DOT officials are not waiting on the “bookends” decision to move forward with plans to overhaul the Exit 28 interchange. Even without the bookends, the widening of Catawba Avenue, finished last year, will carry the DDI flow for a while, Mr. Dunlop said – probably for seven to 10 years, depending on how much growth the area sees in the next few years.
Next, the DOT needs to approach the property owners at the four DDI corners to purchase small sections of land through which the ramps will cut, giving them the right-of-way to proceed with construction plans. DOT officials expect the right-of-way by May 2012, and expect a let date of July 2013. But the let date may get bumped up, NCDOT Engineer John Conforti said.
“If the right-of-way goes smoothly we can move everything up,” Mr. Conforti said, “and we know the town wants to get started as soon as possible.”
VIDEO: DIVERGING DIAMOND INTERCHANGE VISUALIZATION
READ MORE ABOUT THE DDI:
April 18, 2011: “In Mo., officials test a diverging diamond”
March 24, 2011: “New interchange is a ‘sure thing,’ but may not stop there”