Pennsylvania State Route 51 north-south highway. It runs through Central Allegheny County. For the most part it runs along the Ohio and Monongahela Rivers. PA St Rt 51 passes through the following towns in Allegheny County. Jefferson Hills Lebanon Pittsburgh McKees Rocks Coraopolis PA St Rt 51 is multi-plexed in Allegheny County. TRUCK U.S. Rt. 19, at Pittsburg.