By CHRISTINA RITCHIE ROGERS
Plans for a new interchange at the intersection of I-77 and Catawba Avenue are moving forward, and town officials now hope to expand the project to include the two adjacent intersections by the time they break ground in 2013. But a technicality in the language of the federal earmark funding the project may prevent the town from doing so.
In 2004, the town received about $6 million in federal money earmarked for design and engineering of a “diverging diamond interchange,” a unique intersection intended to improve traffic flow and safety for drivers, pedestrians and bicyclists.
With the new interchange, drivers traveling on West Catawba Avenue would still stop at traffic lights on either side of the I-77 overpass – lights that would be the east and west “points” of a diamond shape. When traffic moves, drivers would actually cross over to the other side of the road as they cross the bridge over I-77, then cross back to the regular side of the street, creating a “diamond” shape to the traffic pattern.
“The ultimate goal of a good traffic plan is to keep traffic moving,” Town Traffic Planner Tom Tasselli said. And a diverging diamond interchange does that, as transportation officials have seen in examples such as the one in Springfield, Mo., the first and only U.S. city to build a diverging diamond interchange to date.
In addition to improving the efficiency of an intersection, the interchange design will improve safety by preventing drivers from having to turn left across lanes of incoming traffic, Assistant Town Manager Andrew Grant said.
The interchange also will include a protected lane about 8-feet wide between the two 12-foot driving lanes that will have a short wall barrier and will allow safe passage for pedestrians and cyclists. This feature also is in keeping with the Town Transportation Advisory Board’s goal of increased mobility throughout the town.
CONGESTION: WHAT TO EXPECT
As with most construction projects, drivers should expect some delays while the interchange is being built in 2013.
“There will be more congestion before there is less,” Mr. Tasselli said, for two primary reasons: First, construction will cause some traffic, and second, after the interchange is built, it will take time for people to learn how to properly navigate the intersection.
“It will take getting used to,” he said, “because (crossing into the other lane) is contrary to what you want to do.”
On average, it takes drivers three to six months to get used to a new traffic pattern, he said.
The good news for drivers is the bridge does not need to be widened to accommodate the interchange – a project that would significantly increase construction time and residual traffic.
TOWN OFFICIALS: THE INTERCHANGE IS A MUST
“I don’t see any downsides to the project other than the construction traffic,” Mr. Grant said, and the benefits far outweigh the inconvenience. Mr. Tasselli agrees, particularly when looking at the benefit to traffic plans long-term.
“The DDI is good no matter what,” Mr. Tasselli said. “It will provide immediate relief and the duration of traffic mitigation will be reduced.”
And that relief is necessary, he said.
“The average daily trips are to a point where the amount of time spent sitting at a signal is higher than people will tolerate,” Mr. Tasselli said. If drivers sit for more than two signals, the intersection’s level of service becomes diminished, he said, and the traffic backs up into other intersections.
While the diverging diamond interchange will improve traffic immediately, there is a time limit to its effectiveness, he said, if the construction plans do not consider the adjacent intersections.
“The DDI will be good for 10-12 years,” Tasselli estimates, but if improvements also are made to the “bookend” intersections, its effects would last much longer, he said.
WHAT’S IN STORE FOR THE ‘BOOKEND’ INTERSECTIONS?
Town planners and the N.C. Department of Transportation have been looking at options for improving the efficiency of the nearby intersections on Catawba – at U.S. 21 and at Torrence Chapel Road – either by altering the traffic flow into those intersections or by building roundabouts to keep traffic moving.
Roundabouts allow for a steady flow of traffic and also reduce air pollution, Mr. Tasselli said, because the cars do not sit idle as long as they do at traffic lights. Cars emit the most air pollution while idling, he said.
Another option the town and NC DOT are exploring is putting in a new road through through the Smithville neighborhood from U.S. 21. But, Mr. Grant said, officials still need to study the impact the road would have on the historic neighborhood and nearby businesses.
“We’re very sensitive to that,” Mr. Grant said, “and we’ve let the DOT know they have to be sensitive to that as well.”
Improvements to the bookend intersections may not require any additional federal funding, because the diverging diamond project looks like it will come in under budget. Town officials hope to use leftover funding on the intersections.
“We studied the concept, liked it, determined it would work, and there’s funding that will cover it,” Assistant Town Manager Grant said. “That never happens.”
Construction of the diverging diamond interchange would cost about $3.5 million, Mr. Grant said, which leaves about $2.5 million in earmarked funds that could be put towards the adjacent intersections. But the language of the earmark is such that it designates funds for the diverging diamond plan only, he said.
The diverging diamond is a “sure thing,” he said, but ideas for the adjacent intersections will not materialize if the Town and the DOT can’t get approval from the Federal Highway Administration, that must approve the use of all funds for road construction projects.
“The (Federal Highway Administration) is interpreting the language (of the earmarked funding grant) differently than the town and the DOT are,” Mr. Grant said.
The DOT together with town planners are trying to convince the Federal Highway Administration that upgrading the adjacent intersections is an appropriate use of the earmarked funds as it will enhance the effects of the diverging diamond interchange.
HAVE YOUR SAY
As early as next month, town officials will be seeking citizen comments about the project. Some officials will travel to Springfield, Mo., Apr. 14 to get feedback from citizens who drive through the interchange every day.
“Public input has become very important to projects,” Mr. Tasselli said. “We are the end users and the DOT has come to realize it is possible to work in a way that is sensitive to the community needs as well as to the transportation needs,” he said.
VIDEO: A DIVERGING DIAMOND
Watch an aerial video showing a diverging diamond interchange in use.