A crash-prone, congested highway interchange in Washington County will be the first in the state to have an innovative traffic pattern that crosses some drivers to the left side of the road.
A diverging diamond interchange will replace the traditional cloverleaf at Route 19 and Interstate 70 in South Strabane, as part of an estimated $69 million project to widen a section of I-70 to six lanes. Construction is expected to begin this year and continue into 2017.
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. Leaving the interchange area, through traffic crosses back to the right.
The first diverging diamond interchange opened at I-44 and Route 13 in Springfield, Mo., in June 2009. The Missouri Department of Transportation pronounced it "a huge success," eliminating backups of up to 2 miles and reducing crashes by 46 percent in its first year of operation.
In a survey, 80 percent of drivers said the interchange improved traffic flow and 87 percent said they believed crashes were less likely than in a conventional interchange. Since then, at least 12 other Missouri interchanges have been converted, and more than 30 diverging diamond interchanges are in operation across the U.S.
The I-70 interchange at Route 19 was built in the 1960s. Its traditional cloverleaf design has fallen out of favor 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 headed for the highway on-ramps 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.
A PennDOT report said about 100 crashes occurred in the I-70/Route 19 interchange area from 2006 to 2010.
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. Route 19 will be improved between Oak Spring Road and Trinity Point Drive.
PennDOT expects to accept construction bids in late July. Ramp and utility work is expected to begin this year, with major construction beginning next year, spokeswoman Valerie Petersen said.
Plans will be displayed at a public open house from 5 p.m. to 7 p.m. May 8 at South Strabane Fire Station No. 2, 172 Oak Spring Road.
Jon Schmitz: firstname.lastname@example.org or 412-263-1868. Visit the PG's transportation blog, The Roundabout, at www.post-gazette.com/Roundabout. Twitter: @pgtraffic.