Renovation of I-83 Diverging Diamond Interchange

GFT provided preliminary engineering, final design, and construction consultation for the Interstate 83 (I-83) Exit 4 highway project, which upgraded the existing interchange with PA 851. The interchange area has grown extensively, causing high traffic congestion levels and backups onto I-83. Also, the bridges carrying I-83 traffic over PA 851 were structurally deficient, and the adjacent culvert needed to be replaced. Designing an innovative diverging diamond interchange (DDI) and replacing a culvert adjacent to environmentally sensitive lands highlight the many resourceful features of this “Gateway to Pennsylvania” themed interchange, the first exit for visitors to Pennsylvania traveling northbound on I-83.

The I-83 Exit 4 project is the third DDI built in the commonwealth. Instead of a traditional diamond interchange, the project’s DDI configuration allows vehicles to turn left directly from off-ramps onto PA 851 and from PA 851 to on-ramps onto I-83, eliminating backups. Direct left turns reduce the number of intersection conflict points from 26 in a traditional diamond configuration to 14, which is expected to reduce the crash rate by more than 40%. The DDI has only two phases per traffic signal cycle instead of three, allowing increased capacity and more green time per cycle than a standard diamond interchange configuration to improve traffic flow.

The project team coordinated with the Shrewsbury Wellhead Protection Committee during both design and construction to provide a stormwater management system that would protect the nearby local public water supply. Basin lining and outlet valves were included in the design for spill containment, preventing roadway runoff from infiltrating groundwater in the Deer Creek watershed. Water quality testing was performed before, during, and after earthmoving and pile driving activities.

As a “Gateway to Pennsylvania,” special considerations were made to the Exit 4 interchange landscaping and aesthetics. The project included dry stack stone retaining walls reminiscent of those constructed on nearby farms in the colonial era that support red stone beds filled with seasonal plants. A flagpole with American and Pennsylvania flags was installed in the southeast quadrant, which accompanies artwork placed on the bridge abutments: a 5-foot by 5-foot Liberty Bell and “Life, Liberty, and the Pursuit of Happiness” graphic. Both the flags and artwork are illuminated at night, providing a visual statement in daylight and darkness.