Pennsylvania State Route 121 is a north-south highway in South-Central Allegheny County. It runs competly in Allegheny County. PA St Rt 121 runs from U.S. Rt. 19, near Dormont, to PA St Rt 50, at Pittsburgh. PA St Rt 121 passes through the following towns in Allegheny County. Green Tree Pittsburgh PA St Rt 121 is not multi-plexed in Allegheny County. PA St Rt 121 intersects the following highways in Allegheny County. U.S. Rt. 19, near Dormont. Interstate 376, near Green Tree. U.S. Rt. 22, near Green Tree. U.S. Rt. 30, near Green Tree. Pennsylvania State Route 50, at Pittsburgh.