I-70 to see innovative highway interchange

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An innovative design for highway interchanges that switches traffic to the left side of the road is being planned along Interstate 70 in Washington County.

A "diverging diamond interchange" will be built to replace the conventional cloverleaf at I-70 and Route 19 in South Strabane, the Pennsylvania Department of Transportation announced this week.

In a diverging diamond, both directions of traffic on a main road (in this case, Route 19) are crossed over to the left side on the approach to the highway. That allows left turns onto highway entrance ramps without crossing oncoming traffic. Right turns to entrance ramps are made before the crossover.

The new design will eliminate the existing conflicts between vehicles trying to enter and exit I-70 and provide much longer acceleration and deceleration lanes.

As PennDOT planned a widening project for I-70 in that area, the Federal Highway Administration suggested that it consider the design. "The more we dug into it, the more it seemed like something that would work out there," project manager Barry Lyons said.

The plans will be on display for the public from 5 p.m. to 7 p.m. Wednesday at South Strabane Fire Station No. 2, 172 Oak Spring Road.

As part of the same project, PennDOT plans to widen I-70 to three lanes between Beau Street and the north junction with I-79 and add a second exit lane from westbound I-70 to northbound I-79 at the north junction. The estimated project cost is $65 million and the tentative schedule calls for a contract award in 2014, Mr. Lyons said.

The first diverging diamond interchange in the U.S. opened in Springfield, Mo., in June 2009 and has received overwhelming support from the driving public, according to the Federal Highway Administration. It reduced crashes, improved traffic flow and cost less to build than a more traditional reconstruction.

In a survey by the Missouri Department of Transportation, 97 percent of respondents said they felt safer in the new interchange and a 60 percent reduction in crashes was reported.

Mr. Lyons said 10 more have been built in Missouri, Tennessee, Kentucky and Utah and at least 40 are under design in 19 states and Canada. There are no diverging diamond interchanges in Pennsylvania.

Although having traffic drive on the left side of the road sounds scary, the design of the diverging diamond "makes it pretty much impossible for you to go the wrong way on it," he said.

Another benefit of the design is that the I-70 bridge over Route 19 can be preserved rather than replaced, at a substantial savings, he said.

The I-70 interchange at Route 19 was built in the 1960s. Its traditional cloverleaf design has fallen out of favor with traffic planners because it does not efficiently and safely move heavy traffic. The principal flaw is the conflict with exiting and entering vehicles -- those leaving the highway must merge left from the exit lane in the same area as traffic entering the highway has to merge right.

The on-ramps are very short, giving drivers entering I-70 almost no time to accelerate to the pace of vehicles already on the highway.

The project is one of several major improvements planned or under way on I-70 in Washington and Westmoreland counties. PennDOT District 12 has established a website with project information at www.I-70Projects.com.


Jon Schmitz: jschmitz@post-gazette.com or 412-263-1868. Visit the PG's transportation blog, The Roundabout, at www.post-gazette.com/roundabout . Twitter: @pgtraffic.


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