Auckland city centre and Whenuapai). This motorway is the main connection to the northwest part of Auckland, links to the Waterview Tunnel (thereby being the key route between the city centre and Auckland Airport) and forms part of the Western Ring Route. Upper Harbour Motorway (State Highway 18 between Westgate and Albany). This motorway is the main connection between West Auckland and the North Shore, via the Upper Harbour Bridge. Southwestern Motorway (State Highway 20 between Waterview and Manukau). This motorway forms a large part of the Western Ring Route that bypasses central Auckland, provides access to Auckland Airport and includes the Māngere Bridge and Waterview Tunnel. Because of Auckland's geography and road network layout, many motorways also have arterial road functions and are used for short-distance trips. Access to the motorway network is controlled through ramp signals, which regulate the flow of traffic onto the motorway. While this causes delays and vehicle queues when getting onto the motorway, it produces smoother through flows on the motorway itself. Major arterial roads Around 16% of Auckland's overall road network is classified as either a primary or secondary arterial road. These roads generally have a speed limit of 50 km/h in urban areas and serve a wide variety of functions, providing for the movement of people, goods and services as well as public transport services, cycleways, on-street parking and access to businesses and residences. To best balance their wide variety of different functions, at peak times some roads have bus or carpool lanes, while on-street parking is frequently restricted. Key infrastructure Key bridges, tunnels and interchanges are critical to the functioning of Auckland's road network. Major pieces of infrastructure include: Auckland Harbour Bridge. Opened in 1959, this bridge is 1,020 m (3,348 ft) long, with a main span of 243.8 m, rising 43.27 m above high water. It forms part of the Northern Motorway and is the critical link between the North Shore and central Auckland. The bridge was extended in 1969 by constructing cantilevered 'clip-on' lanes at both outer sides of the original construction, giving it 8 lanes in total (from 4 lanes before). As this eventually was still not enough to manage growing traffic loads, a traffic management solution involving a movable barrier was installed, giving it 5 lanes per dominant direction depending on the time of day. Upper Harbour Bridge. This bridge in the northwest of Auckland is an