Route 378 continues northward through Polanski Park and crosses a Norfolk Southern railroad line before crossing the Lehigh River on the Hill to Hill Bridge.
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In 2009, the portion north of the Lehigh River was named the Fred B. Rooney Highway.
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PA 412 intersects an access road to the Hill to Hill Bridge, which carries PA 378 over the Lehigh River to the north, at which point PA 412 turns south onto Brodhead Avenue.
Upper Saucon has three north-to-south numbered routes: 309, 145, and 378. In Lanark, 309 comes north from Philadelphia via Quakertown to join Interstate 78 coming east from New York City and cross the mountain to Allentown.