Because of vehicle weight limits, the MacKay Bridge is the only crossing that commercial trucks can use to cross the harbour. The bridge feeds into Highway 111, the Circumferential Highway. It is located to the north of the Macdonald Bridge, at the Narrows of Halifax Harbour. Murray MacKay Bridge, known locally as the MacKay Bridge or the 'new bridge', was opened on July 10, 1970. Other than transit buses, no vehicles weighing over 3,200 kg (7,100 lb) are permitted on the Macdonald Bridge vehicles in excess of this weight limit must cross the harbour via the MacKay Bridge. It is only the second time ever that the suspended spans of a bridge have been replaced while being open for traffic during the day the first being the similar Lions Gate Bridge in Vancouver. The only original components are the main cable, the two towers, the anchor piers and cable bents. From 2015–2017, the bridge structure suspended below the main cables was completely replaced, including the floor beams, stiffening trusses, road deck and suspender ropes. The Macdonald Bridge often suffered from traffic problems due to its narrow (40 ft, 12.2 m) width, so a lane addition project was completed in 1999, relocating the pedestrian and bicycle outboard of the main cables. The bridge was designed by Phillip Pratley, a bridge designer also responsible for the Lions Gate Bridge in Vancouver. Macdonald Bridge, known locally as the Macdonald Bridge or the 'old bridge', was opened on April 2, 1955. The commission's toll rates are approved by the Nova Scotia Utility and Review Board. The commission has nine board members, five of whom are appointed by the provincial government, including the chair and vice-chair the Halifax Regional Municipality appoints four councillors to the remaining seats on the board. The Commission reports to the Minister of Finance and the Nova Scotia Cabinet approves the Commission's financing. In March 2009, the commission launched its current operating name, Halifax Harbour Bridges, as well as an updated logo. The commission also maintains authority over any other bridge or tunnel structures crossing the harbour limits, including the Northwest Arm, although no such bridge or tunnel currently exists. The commission is responsible for constructing, operating, and maintaining two suspension bridges that cross Halifax Harbour between the communities of Halifax and Dartmouth. It currently operates under a new statute passed in 2005 named the Halifax-Dartmouth Bridge Commission Act. The Halifax-Dartmouth Bridge Commission, operating as Halifax Harbour Bridges, is a Nova Scotia Crown corporation created in 1950 by provincial statute.

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