Bechtel and Parsons Brinckerhoff jointly supervised the planning and construction that re-routed and rebuilt the Central Artery from an elevated roadway to a series of tunnels.[11] Planning began in 1982 and construction lasted from 1991 to 2006, approximately ten years longer than initially planned.[12] Also known as "Big Dig", the project included construction of the Leonard P. Zakim Bunker Hill Memorial Bridge, the widest cable-stayed bridge in the world,[11] and an underwater tunnel that traverses the Boston Harbor and connects downtown Boston with Logan International Airport.[11] The megaproject was considered the most complex and expensive highway project in United States history.[12][13]