Gas Transmission mileage in the United States predominantly skews toward mid-20th-century installations. Plot 1 presents the most recent Gas Transmission mileage data from PHMSA’s By-Decade Inventory. Plot 2 takes this same data and presents the changes to mileage since 2010. The numbers on each side of the area plot denote each decade’s 2010 and 2021 mileage. This chart displays the gradual decline of pre-2010s pipeline mileage and the gradual increase of the latter. This point on broader system age composition is further explored in Plot 3, where post- and pre-1970s pipeline are compared. The incident rates of these pipelines are further examined in our analysis of Incident Rates by Age.