before entering Beaver County. The route spends 36 miles in Allegheny County. At the Beaver County line, PA 51 intersects the eastern terminus of Pennsylvania Route 151. North of PA 151, PA 51 enters South Heights as Jordan Street. The route extends past the Ambridge-Aliquippa bridge, through the city of Aliquippa and West Aliquippa. Farther to the north, the highway heads towards the north as Constitution Boulevard entering Monaca as Beaver and Pennsylvania Avenues before making a 90-degree turn towards the north. PA 51 crosses the Ohio River on the Monaca-East Rochester Bridge, entering the borough of East Rochester. It makes a 90-degree turn to the west to begin a concurrency with Pennsylvania Route 65. In Rochester, PA 51/PA 65 meet with Pennsylvania Route 18; the concurrency ends in downtown Rochester, when PA 51 begins a concurrency with Pennsylvania Route 68. PA 51/PA 68 cross the Beaver River on the Beaver Bridge. In Bridgewater, the PA 51/PA 68 concurrency ends when PA 51 makes a 45-degree turn towards the northwest and continues towards the northwest as Constitution Boulevard. In Chippewa Township, it intersects Interstate 376 (James E. Ross Highway) at exit 31. Northwest of I-376, Route 51 intersects the western terminus of Pennsylvania Route 588 and Pennsylvania Route 251. In South Beaver Township, PA 51 intersects Pennsylvania Route 168 and continues towards the northwest as Constitution Boulevard. PA 51 terminates (ends) at the Ohio state line; in Ohio, it continues as Ohio State Route 14. History Route 51 was first signed in 1927. The north section was signed first, followed by the section between Pittsburgh and Uniontown. Major construction projects included a bypass of Perryopolis built in 1950, the West End Bypass in Pittsburgh that was completed in 1953, and the interchange at the southern portal of the Liberty Tunnels in Pittsburgh. Unlike PA 88 and PA 28, Route 51 wasn't truncated in 1961 to reduce concurrencies, making it the only major state route (not including U.S. routes or Interstate routes) to still pass through Pittsburgh following the separation of PA 65 and PA 50 from PA 88 & PA 28, respectively; PA 51 serves as a link between PA 88 & PA 65 and, via PA 60, I-376, & I-279, a link between PA 28 & PA 50. Before the turnpikes were built, Route 51, alongside Ohio State Route 14, served as the main corridor connecting Pittsburgh and Cleveland.