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York County’s top ranked I-77 traffic site may not get its diamond. So, what’s a SPUI?

SCDOT talks I77 and SC 160 fix in Fort Mill

RFATS and SCDOT discuss I77 and SC 160 fixes in Fort Mill. Options are a diverging diamond and a SPUI.
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RFATS and SCDOT discuss I77 and SC 160 fixes in Fort Mill. Options are a diverging diamond and a SPUI.

It sounds like a bowl of noodles and looks a bit like one too. But drivers in one of the worst traffic spots in York County soon may get a taste of just what a “SPUI” is.

The question is, will it help?

Berry Mattox, project manager with South Carolina Department of Transportation, outlined several options for the coming interchange at I-77 and S.C. 160 in Fort Mill. That site was long-considered by many for a diverging diamond similar to one approved at I-77 and Gold Hill Road.

“The (diverging diamond) is still out there,” Mattox said. The (diverging diamond) works, and was the preferred alternative at Gold Hill. So certainly that is an alternative that’s being vetted here.”

Other, costlier yet potentially better, options are also in consideration. They’re variations of a single point urban interchange — SPUI — involving new bridges across the interstate, new lanes, and fewer left turns.

The state agency hasn’t decided which option to take. Public comment is still required. A public meeting will be scheduled in the fall to present alternatives. Here are the pros and cons of the viable options in consideration:

Diverging Diamond

The N.C. Department of Transportation made this video to explain the "diverging diamond" interchange or DDI. The department is planning to build two of them in the Triangle, at the Beltline and Western Boulevard in Raleigh and at Interstate 40 and

While some may already have assumed a diverging diamond was the choice, road experts say it always was presented as a potential fix for the intersection that swells with traffic from Baxter on one side and Kingsley the other.

“We had put the concept of a (diverging diamond) out there as one alternative...,” said David Hooper, director of the Rock Hill-Fort Mill Area Transportation Study.

The diverging diamond includes two traffic signals on either side of the existing S.C. 160 bridge over I-77. Cars would switch sides as they across the bridge to create free flow left turns on and off the interstate. Fewer left turns is the larger concept guiding road improvement decisions throughout high traffic areas in Fort Mill, Rock Hill, Tega Cay, Indian Land and Lake Wylie.

A diverging diamond would improve travel efficiency at I-77 and S.C. 160, leaders say.

“It’s a much more efficient signal setup so you have less yellow time and less red time and the signals operate much better than a typical traffic signal with four and five and six and eight phases,” Mattox said.

A pitfall is, it puts a signal closer to the existing one at Assembly and Carolina Place drives compared to other alternatives.

Single Point Urban Interchange

The best way to describe a SPUI is to point 10 miles north.

The Tyvola Road interchange in Charlotte directs traffic to one signal, with a variety of patterns flowing into and out of it.

“Since we have one traffic signal, it brings all the movement together and so your left turns will run with these other left turns,” Mattox said. “It was kind of the flavor of the week 10, 15 years ago. It seemed to be what was popping up everywhere.”

Like the diverging diamond, a central SPUI would improve conditions. It would, though, require extensive bridge work almost to the point of bridge replacement. It also would put that large, single traffic signal about 800 feet from the existing Baxter signal.

Close signals coming off interchanges cause problems, as RFATS leaders note regularly with another high-traffic area at I-77 and Celanese Road in Rock Hill. Traffic there is to a point where another bridge crossing the Catawba River has been discussed at length. Intersection improvements are ongoing in the area, with leaders saying a traffic light may have to be removed if they don’t work.

Offset SPUI

Getting the benefit from a single light without the signal being so close to the Baxter signal would mean, obviously, moving the SPUI signal.

“That’s the big win there,” Mattox said of one of several offset options. “We previously had 800 feet. Now we’re at 1,500 feet.”

An offset SPUI would mean two new bridges crossing I-77 to eliminate left turns. Bridge size, lane paths and other details vary.

The most intensive option is a full directional interchange with flyover routes similar to what drivers might expect to see at interchanges in large cities.

By offsetting the large intersection, less traffic would flow across the existing S.C. 160 bridge compared to a typical SPUI. It wouldn’t require bridge replacement there. Also, the new flyover bridges could be built with minimal disruption.

“We can essentially build this under traffic without really modifying any of the existing patterns,” Mattox said. “The interchange itself could work as it does now while we construct these two bridges. Really you could almost shift it over on a weekend.”

More paths and more bridges across the interstate, though, mean a progressively higher price tag.

The cost

Costs increase from diverging diamond to SPUI to offset SPUI, but they aren’t set.

All options would be higher than prior mid-$20 million estimates to improve the interchange and $33 million for that work plus road widening.

Hooper estimates the diverging diamond, along with the widening of S.C. 160 to six lanes with a divided median as planned, could cost in the mid-$30 million range.

“Some of these configurations could very easily put you in the low $50s,” he said.

Mattox estimated a little higher, at around $40 million on the low end for the entire project up to more than $60 million depending on the configuration picked. All for a project still several years away from construction, aimed to improve traffic for up to 25 years.

“Cost is a consideration,” Mattox said, “but it can’t be the determining factor.”

Environmental and right-of-way impacts have to be considered. Road leaders have to show whatever they pick will improve traffic. Which could mean a more expensive route.

“We can’t let cost drive the project,” Mattox said. “We have to demonstrate first and foremost that what we’re going to do is going to work.”

Even with support from state officials up to the governor’s office, Hooper said, getting the money for work at S.C. 160 could mean borrowing federal money from groups similar to RFATS.

RFATS could have to borrow several years’ worth of funding. York County has an application to the state infrastructure bank that also could bring money.

Other options

RFATS and other road planners have talked about a new interchange north of S.C. 160, at Coltharp Road.

“We’ve had prior discussions about the advisability of having something there,” Hooper said. “That it seems to make sense to incorporate something given what may happen at the (former) Knights Stadium, full buildout at Kingsley. That’s still an evolving picture.”

For now, Coltharp doesn’t make the cut.

“When they ran the numbers, it didn’t provide enough benefit to go ahead and go forward,” Hooper said.

Another option is to wait. It isn’t a popular one.

Chief Bill Harris with Catawba Indian Nation, an RFATS policy committee member, sees air quality and other benefits to the more expensive options since they would keep more traffic moving. He sees those options as planning for expected growth to come.

“If we don’t do it now, in 10 years we’ll come back and we’ll have this conversation — or somebody will be having this conversation — but now the price tag has grown,” Harris said.

Only going so far

For all the planning effort, the interchange improvement can do only so much. Someone driving from Rock Hill into Charlotte for work each morning may not feel the fix. Kingsley businesses and residents will.

“It will not do anything on I-77,” Hooper said. “It will help process the people in and out of the interchange. The backup going north in the morning doesn’t get better because of this. What it does is it helps clear this area.”

The work won’t fix the Assembly and Carolina Place signal, which can’t run concurrently now because of a grade difference in elevation.

“Which is very inefficient, and the signal does not operate particularly well because of that split phasing, which is necessitated because of the geography out here,” Mattox said.

When York County submitted an application to the state infrastructure bank for help funding interchange improvements from Carowinds Boulevard in Fort Mill to Cherry Road in Rock Hill, the S.C. 160 project ranked No. 1. Still, committing so much money to that interchange means less money for other needs.

“It’s becoming sharper, the results of our decisions,” Hooper said.

RFATS borrowed money for past projects and it’s “within our ability...” to do it here, he said. Yet with so much growth happening in so wide an area of York and Lancaster counties, the S.C. 160 project highlights a need for change in thinking about roads.

“Traditional approaches aren’t keeping up with the demand within the region,” Hooper said. “We’re having to get ever-more elaborate to make it work because of just how fast the growth is and the amount of demand we’re trying to process down our major corridors. And this is a prime example of it.”

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