Update desk: DOT mulls fixes to I-85

By Cindy Landrum  

MAY 5, 2011 10:45 a.m. (0)

One of the solutions being studied by the South Carolina Department of Transportation to ease congestion along a 22-mile stretch of Interstate 85 in Spartanburg and Greenville counties is being used at five sites in the country.

Diverging diamonds temporarily put drivers on the left side of the road at interchanges to eliminate the need for drivers to make left turns in front of oncoming traffic.

“It’s a radical approach,” said Michael Dennis, project engineer with the state DOT.

Columbia-based Civil Engineering Consulting Services has been studying options to alleviate congestion from White Horse Road in Greenville County to State 129 in Spartanburg County.

The stretch of interstate is a key transportation artery for tractor trailers supplying some of the Upstate’s largest industrial plants and distribution centers, for Upstate residents working at them and other nearby businesses and for traffic passing through on the way to Atlanta or Charlotte.

The options being studied are divided into four areas: travel demand management, modal, operational and capacity, Dennis said.

A final report is expected in June, giving the DOT a “buffet” list of options to choose from for the $9 million it has budgeted for I-85 improvements, he said.

One of the options is installing at the State 290 interchange a diverging diamond, a popular option in France, but one that has only been used in three states, Dennis said.

The first one was built in Springfield, Mo., in 2009. Missouri has three diverging diamonds, while Utah and Tennessee have one each, according to a recent article in

“Time” magazine.

In a diverging diamond, traffic lights allow lanes to crisscross at an intersection – allowing drivers to make left turns without having to negotiate oncoming traffic – before putting them back in their regular lanes of traffic.

Drivers making right turns are still allowed to do so.

Dennis said the new configuration has been shown to reduce traffic accidents at those interchanges by 50 percent.

“It takes the left turn conflicts away,” he said.

And according to a Federal Highway Administration report, the interchanges can handle twice as many left turns per hour as conventional interchanges, reducing delays for drivers.

Using diverging diamonds can increase traffic flow without having to build additional lanes of traffic, something transportation officials said could cost hundred of millions of dollars.

Don Saiko, the project manager for the nation’s first diverging diamond, told “Time” that he expects diverging diamonds to be the “next big thing” in highway construction because of its lower cost. He told the magazine it cost $3.2 million compared to $10 million to expand the intersection.

Diverging diamonds were named one of the best innovations of 2009 by “Popular Science” magazine.

Among other options expected to be included in the final report are “park and ride” areas, an express bus along U.S. 29, more overhead message boards, bus rapid transit to the airport, extending ramps, widening some portions of I-85 and having collector-distribution systems similar to the one for northbound traffic around State 291, Mauldin Road and Augusta Road that keeps traffic from having to get on the interstate only to get off in an exit or two, Dennis said.

“We want to extend the life of the corridor,” he said.