Pennsylvania Route 18 is a major north–south highway in Western Pennsylvania whose southern terminus is at West Virginia Route 69 at the state line in Greene County near the village of Garrison, while the northern terminus is at PA 5 in Lake City. At a length of 205 miles (330 km), PA 18 is the only state route in Pennsylvania — north–south or east–west — to traverse the entire state. It also has the distinction of being the longest state route in Pennsylvania.

Greene County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 35,954. Its county seat is Waynesburg. Greene County was created on February 9, 1796, from part of Washington County and named for General Nathanael Greene. The county is part of the Southwest Pennsylvania region of the state.