Time to pick a route at I-77 and S.C. 160 exit. Here are options, pros and cons.
Project manager Berry Mattox says any of the three options to fix traffic congestion at I-77 and S.C. 160 will work. What he wants to hear is, which fits best?
The South Carolina Department of Transportation has a public meeting set for 5-7 p.m. Jan. 30 at Real Life Church on Pleasant Road. Mattox and his team will have displays on the three ways they could improve the interchange. But with varying costs, advantages and challenges, road planners aren’t pushing any of them to the front of the line.
“We’re going into this with eyes wide open,” Mattox said.
Ahead of Thursday’s meeting, Mattox took time to break down some of the differences for a public that may not know a diverging diamond from a single point urban or directional interchange.
Diverging Diamond
This alternative puts two traffic signals in, one on each side of the S.C. 160 bridge over I-77. Traffic on that bridge would swap sides of the road to create off ramp style right and left turns.
“That’s where we kind of shift traffic to the opposite side,” Mattox said. “You’re going the wrong way, it appears.”
Advantages: A diverging diamond likely is the least expensive option (all cost estimates are ballpark figures right now, Mattox said), and it simplifies traffic signals into two phases.
“It’s a very simple, efficient traffic phase,” Mattox said. “It eliminates the competing movement.”
The diamond takes out the traffic overlap of interstate users coming from Baxter/Lake Wylie and Kingsley/Fort Mill sides. Pedestrians could move through the middle or outside of the interchange depending on how many bridges there are, and bicycles could have a separate path or ride with slower-moving traffic.
Familiarity with a diverging diamond may not be high now for area drivers, but it could be by the time work finishes. The I-77 and Gold Hill Road interchange under construction, too, will be a diverging diamond.
Challenges: One issue known to cause traffic backups at interchanges is poor spacing between signals. The diamond doesn’t address that concern.
“The signal spacing is not great,” Mattox said.
The signal between Market Street and the bridge sits closer than planners would like to the Carolina Place and Assembly drives signal. Traffic on the bridge also would slow to about 25 mph, and the bridge deck would need widening or a construction of a new bridge.
Construction will take more involved planning since crews will have to maintain traffic while working where it normally flows.
“All the construction is taking place where the cars currently are,” Mattox said.
Cost Estimate: $25-30 million
SPUI
A single point urban interchange, or SPUI, puts some of the same concepts from the diverging diamond into one traffic signal. The nearest, best example of a SPUI is the exit 5 interchange at Tyvola Road and I-77 in Charlotte.
“You take both of those traffic signals and you kind of collapse them into one signal,” Mattox said.
Advantages: A signal in the middle of the S.C. 160 bridge increases spacing between signals by more than 1,000 feet. Lane movement to Market Street would improve. All non-right-turn traffic would flow to a single signal which could simplify the experience for drivers. Drivers wouldn’t swap to the left side.
“It would increase our spacing,” Mattox said.
A SPUI is the mid-range option in terms of price.
Challenges: A SPUI would require a new bridge. It also uses eight traffic phases at the light to get vehicles going in all possible directions, so drivers could sit at the single light longer. The SPUI doesn’t eliminate the conflict of drivers coming from opposite directions to use the interstate.
“This does not necessarily create the most efficient reconfiguration of those different movements,” Mattox said. “You’ve got those competing.”
The pedestrian path would be lengthy, and cyclists could be on a separate path to the outside.
Cost Estimate: $35-45 million
Directional
The third option takes the single light concept and shifts it to the Kingsley side of the interstate.
“We’ve kind of moved it to one side,” Mattox said.
Advantages: Shifting the light puts it even farther from the Carolina Place/Assembly intersection than the SPUI would. It separates overlap traffic from each side during morning and evening rush hours. All traffic getting onto I-77 could happen via a right turn onto a ramp.
Pedestrians and cyclists could have shared use paths on one side of the interchange. Much of the work required would happen independent of the current roadway.
“It can be built with minimal disruption to traffic,” Mattox said.
Challenges: The directional route would be most expensive, perhaps twice the cost of a diverging diamond.
“It would require two new bridges over I-77,” Mattox said.
Northbound interstate drivers coming from the Baxter side would have to use a new flyover lane, and southbound interstate users coming from Kingsley would use a circle ramp joining up with I-77 under the existing bridge. A local though larger example would be the I-77 and I-485 intersection in North Carolina, two interchanges north of Carowinds.
Cost Estimate: $45-$60 million
The decision
Mattox said it’s common to have a public meeting where a road design is all but set, more an introduction of the project than anything.
This project is different, he said. His department isn’t leaning toward any of the three options.
Interest has been high, he said, and road planners expect perhaps a couple hundred people at the public meeting. There won’t be a formal presentation. SCDOT leaders will listen to questions and feedback, and will further detail the options.
Mattox said anyone who can’t make the meeting can get information or submit comments through the project website at sc160project.com. Online comments will carry the same weight as others offered at the public event, Mattox said.
Still, with an often clogged interstate right in the middle of Baxter, Kingsley, fast-growing areas like Fort Mill, Tega Cay and Lake Wylie, Mattox expects plenty of folks will want to come out and have their say Thursday.
“We’re expecting a crowd,” he said.
This story was originally published January 29, 2020 at 12:38 PM.
