North view from the U.S. 322 abutment at the new SR 2015 bridge over Potter Run and gap earthwork.

Potters Mills Gap Transportation Project

Carving an expanded highway through a mountain gap in Central Pennsylvania

Our Client’s Challenge

Potters Mills was a well-known bottleneck to travelers of U.S. Route 322 in Central Pennsylvania that connects Mifflin and Centre Counties. Increased traffic flows along the narrow and steep two-lane portion of U.S. 322 through the gap caused impactful safety concerns and traffic backups.

Our Solution

PennDOT Engineering District 2-0 selected Gannett Fleming for project design and construction consultation for building the new 3.75-mile roadway, including adding four new bridges, inserting new retaining walls, and relocating a portion of the highway into the mountainside. Our firm managed the overall project, including highway and geotechnical design, permitting, and construction services.

The project replaces a winding section of U.S. 322, from west of the Pennsylvania Route 144 intersection to the Seven Mountains area near the Centre and Mifflin County line. The final design incorporated new interchanges at both ends of the corridor, widening the existing two-lane U.S. 322 highway to a four-lane limited access facility, and constructing a parallel frontage road (Pennsylvania State Route 2015).

Design challenges included fitting four mainline lanes and two frontage road lanes in the gap between the mountainside and Potter Run stream. To solve this complex problem, three large retaining walls and innovative stormwater management best practices, including ultra-extended detention basins, were built in the gap.

The roadway had several sharp curves and steep vertical grades ranging from just over 1% to over 8%. The highway’s alignment was carefully designed to meet current highway criteria, while minimizing impacts on the surrounding natural environment. The design team used the maximum allowable flexibility with vertical grades and horizontal curvature to conform the roadway as closely as possible to the topography. A bifurcated vertical alignment through the gap reduced the excavation height, excavation volume, and roadway footprint.

Most stream and wetland areas required compensatory mitigation alignment that minimized the impact on the surrounding forestlands and streams. This was accomplished with a bifurcated vertical alignment, retaining walls, and soil nailing. Thus, a new stream channel was created and is now an unnamed tributary to Potter Run. The stream design includes riffle beetle habitat structures (with embedded logs), floodplain benches, and minimized rock on the streambanks.

Key Features

  • Four mainline lanes and two frontage road lanes in the gap.
  • Three large retaining walls and ultra-extended detention basins.
  • Four reconstructed bridges.
  • New roundabout at the westbound off-ramp of Old Route 322.

Awards & Recognition

  • Awards. This web part is hidden.


  • Improves safety, reduces congestion, and enhances mobility.
  • Minimizes future maintenance with a 100-year structure design.
  • Protects wildlife through enhanced stream and watershed restoration.


Pennsylvania Department of Transportation (PennDOT) Engineering District 2-0


Potters Mills, Pa.



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