Traffic snarls every day at rush hour as cars head for Johns Island, lining up on U.S. Highway 17, waiting to make a left turn onto Main Road.

The congested intersection has such a high rate of crashes that the state Department of Transportation's Safety Office proposed using federal money to contribute $2 million toward the cost to improve it.

The DOT plans to rebuild the intersection using an unusual new design called a “superstreet.” Although the design appears confusing, DOT officials said signs would direct drivers through the intersection, and traffic would flow more efficiently and safely.

Charleston County Council on Tuesday approved contributing the additional $1.5 million necessary to complete the intersection, with the money coming from future income from the county's half-cent sales tax,

A “superstreet” intersection keeps traffic flowing more freely on a major thoroughfare by reducing the effect of left turns and crossings from a side street, said project manager Keith Riddle, a safety projects engineer for the DOT.

Under the design, traffic from Main Road won't be able to turn left or cross U.S. 17. Instead, cars must first turn right onto U.S. 17, move into the left lane, then make a U-turn.

“It removes the conflicting turns,” Riddle said, “and it lowers the potential for T-boning.”

Molly LeMin, a project manager in the county's transportation department, said the project marks the first “superstreet” in the state. The model has been successful improving traffic flow and safety in other states, she said, including North Carolina. There's a “superstreet” along U.S. 17 in Leland, N.C., that is working well, she said.

Such intersections not only are safer but also “drastically reduce delay times,” she said.

Riddle said the U.S. 17 and Main Road intersection was selected for improvements because of its high accident rate. There were 359 crashes, three fatalities and 193 people injured there between 2003 and 2011, according to the most recent data available, he said.

At the intersection of Main and Old Charleston roads, which sits just 300 feet from the U.S. 17 and Main Road intersection and adds to congestion and safety issues, there were 105 crashes and 53 people injured.

Riddle said construction can't begin until financial, environmental and right-of-way matters have been settled. He said he hopes the intersection will be complete by the end of 2014.

It will improve safety by reducing the number of crashes and reduce congestion, he said. And it will be much less expensive than an intersection with overpasses, which was another option for improving traffic flow.

Elliott Summey, County Council's vice chairman, said he became aware of the severity of problems with the intersection during discussions on the completion of Interstate 526. Many Johns Island residents said improvements were needed.

The county will pay for its portion of the intersection with left-over money from projects funded by the half-cent sales tax program that came in under budget, Summey said. The county also expects the sales tax program to generate more money in future years than it has in the past few years.

County Administrator Kurt Taylor said the county won't have to provide its share until after the DOT contributes $2 million. So there is time to generate the money from a variety of sales tax sources. “We'll be able to pull it together,” he said. “We're confident we can.”

Reach Diane Knich at 937-5491 or on Twitter @dianeknich.