Potomac prospered into the late 20th century, with a population peaking at 900 during the 1970 census, due in part to its location along US Route 136, one of the primary East-West highways crossing the state of Illinois. The opening of Interstate 74 reduced the amount of travel through Potomac, leading to a rapid decline in the number of service stations, restaurants and small businesses in the downtown area in the early 1970s. In the early 1990s a fire destroyed several downtown buildings on the north side of Route 136, dramatically altering the appearance of the community.