Just over ten miles long, Maryland Route 509 is found entirely in Port Republic, Calvert County, and was constructed from east of MD 765 in the early 1930's, and it was extended west to MD two and MD four in the early 80's
Known for most of its length as Governor Run Road, the state highway runs 1.14 miles (1.83 km) from MD 2 and MD 4 east to a dead end at the Chesapeake Bay within Port Republic. 
