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Interstate 70, Section T20 Reconstruction and Widening

Pennsylvania’s first diverging diamond interchange improves safety and mobility along a heavily traveled corridor.
  • Aerial view of the new diverging diamond interchange connecting I-70 and Route 19 in Pennsylvania - Gannett Fleming.

    A diverging diamond interchange (DDI), the first in Pennsylvania, connects I-70 and Route 19.

  • Ground-level view of intersections at the new interchange - Gannett Fleming.

    By reducing conflict points compared to other interchanges, the DDI is expected to improve safety.

  • Ground-level view of the crossover south of I-70 showing traffic lights and signing at the intersection - Gannett Fleming.

    Crossovers have two-phase signals and allow for free-flow left turns onto I-70 entrance ramps.  

Pennsylvania Department of Transportation
Golden Triangle Construction Company, Tri-State Design and Development

Washington County, Pennsylvania

Our Role
Preliminary Engineering, Final Design, Construction Consultation

1.5 miles
Construction Cost
$64 million
10 years, 6 months
  • DDI has 12 fewer traffic conflict points than a traditional diamond interchange
  • Added travel lane in each direction means less congestion and improved safety
  • Minimal impacts to private property, including commercial development
  • Vehicle access maintained during construction
  • Saved millions by avoiding dual three-span mainline bridge replacement.

The first interchange of its type in the state is helping the Pennsylvania Department of Transportation (PennDOT) achieve its goal of increasing safety and mobility along the Interstate 70 (I-70) corridor. As part of a long-term improvement project, PennDOT reconstructed a 1.5-mile stretch of the highway in Washington County. The section is located within the portion of the interstate shared with I-79. The project included reconfiguring the functionally obsolete Route 19 cloverleaf interchange, which could not be upgraded to current standards without significant commercial property and right-of-way costs. A combined daily average of 98,000 motorists use I-70 and Route 19 in the project area, which is surrounded by commercial development.

PennDOT’s goal was to rebuild the interstate and interchange to improve the degrading highway, increase safety, and reduce congestion throughout the I-70 corridor, while minimizing property impacts and traffic disruption during construction. Gannett Fleming designed a diverging diamond interchange (DDI) to make the I-70 connection with Route 19 safer and more efficient. The team also designed the widening of I-70 for an additional third travel lane in each direction to increase capacity. With the addition of a second lane on the I-79 northbound ramp along with the third lane on I-70, a major fork was created at the I-79 North Junction interchange, where the I-70 center lane is a decision lane.

What We Did

The design and implementation of the DDI means that traffic now crosses over to the left side of the roadway within the interchange. It also means that left turn movements from Route 19 to the I-70 entrance ramps are free flowing, occurring from the far left side of the roadway, and left turns from the I-70 exit ramps are directly into the Route 19 travel lane. Because of these features, traffic conflict points are reduced to 14 compared to 26 in conventional diamond interchanges. The DDI also reduces traffic platooning on I-70 entrance ramps. To adapt the configuration to the local topography, the design team provided a 2 percent superelevation to Route 19, which was on a reverse curve alignment. The team also developed a traffic control plan that kept vehicles moving safely along I-70 and through the interchange as it was being rebuilt. Because the DDI fits primarily within the old interchange footprint, right-of-way impacts were minimized, and no businesses were displaced.

The design retained the existing I-70 bridges over Route 19, saving millions of dollars by avoiding replacement. Both bridges were redecked to extend their service lives. The westbound bridge was raised to increase the vertical underclearance from 14 feet, 2 inches to 16 feet, 6 inches. Improvements to I-70 included widening the highway to four lanes in each direction between the Route 19 and Route 136 interchanges, which provides auxiliary lanes between the interchange ramps. A new decision lane at the widened I-79 North Junction interchange offers motorists easier access to I-70 westbound and I-79 northbound. All of the improvements result in a safer trip for tens of thousands of motorists every day.

Key Features

  • Third travel lane in each direction for added capacity on I-70, with a fourth lane added between the closely spaced Route 19 and Route 136 interchanges, improving traffic flow
  • To sustain its longevity, I-70 was replaced with a concrete pavement structure expected to extend the service life of the interstate system
  • A new major fork condition created a decision lane along I-70 westbound approaching the I-79 North Junction interchange, which improves traffic flow
  • Existing bridges over Route 19 were redecked to extend service lives, provide bridge barriers that meet interstate safety requirements, and avoid costly mainline bridge replacement
  • Widened the existing I-70 westbound bridge over Route 1009 (Locust Avenue) adds third travel lane for increased capacity
  • Decorative stone in ramp island areas reduce the amount of impervious surface in the project, which prevents flooding and lessens stormwater runoff
  • Aesthetic treatments include colored concrete in islands and median areas. 

Awards & Recognition

  • No. 7 Road, 2017, Roads & Bridges, Top 10 Roads
  • Excellence in Concrete Award, 2017, PA Aggregates and Concrete Association (PACA).

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