Engineering Project Case Studies | Erdman Anthony

Diverging Diamond Interchange

Improving Traffic Flow at a Southern Pennsylvania Interchange

Interstate 83 Exit 4 in Shrewsbury, Pennsylvania, was experiencing significant increases in traffic volumes due to commercial and residential growth in the area. The high traffic volumes caused vehicles to queue on the off ramps, and the queues sometimes extended onto the interstate, creating safety concerns. In addition, the I-83 bridge over SR 851 was structurally deficient, and there were challenges involving the surrounding land, including dense local business use, wetlands, and steep terrain.

To bring the exit up to current traffic safety and capacity standards, PennDOT chose to pursue a full replacement of the interchange and the bridge. The location was determined to be a good fit for a diverging diamond interchange, with the resulting project being the third interchange of this type to be built in Pennsylvania.

As part of the project, the existing I-83 bridge over SR 851 was replaced with a wider structure to accommodate additional lanes. The diverging diamond interchange constructed on SR 851 now has two eastbound and two westbound lanes, with traffic signals at the bottom of the ramps. Traffic crosses left and under the bridge, creating smooth transitions on and off the interstate.

Eco-friendly aspects of the project included replacing an existing metal pipe with a concrete box culvert built with natural streambed material to support aquatic life traveling Deer Creek; enhancing the adjacent stream area; and using a 100-year standard of concrete for the culvert.

The project design incorporated aesthetic enhancements such as earth-tone dry-stacked stone retaining walls with flower beds along the bridge and bioretention basins adjacent to the bridge ramps. 

Erdman Anthony provided construction management and construction inspection services for the new interchange, including confirming that the contractors carried out contract requirements, such as environmental commitments made by PennDOT. To assist construction inspection staff with this complex project, one of Erdman Anthony’s senior construction inspectors spent several months creating a small, to scale model of the project. 

While this project presented some unique challenges, the interchange was completed on schedule, and the finished product has provided the community with an exit that is safe, efficient, aesthetically pleasing, and environmentally conscious. 

Sustainable Design Impact

Improvements to adjacent stream, replacement of metal pipe with 100-year standard concrete culvert lined with natural streambed materials.


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2022 TQI Partnering Award
Associated Pennsylvania Constructors
Noel Caron, PE
Noel Caron, PE
 717-766-1741 x 5009