Cabin John Parkway is an automobile parkway in the U.S. state of Maryland. The parkway runs 1.50 miles (2.41 km) from Clara Barton Parkway in Cabin John north to Interstate 495 (I-495) in Bethesda in southwestern Montgomery County. Cabin John Parkway is a four-lane freeway that serves as a connector between Clara Barton Parkway in the direction of Washington and I-495 in the direction of Rockville and Silver Spring. The parkway was constructed in the mid-1960s and passes under the historic Union Arch Bridge, the longest masonry arch span in America.