State Highway 20 (SH 20), also known as the Southwestern Motorway, is a New Zealand state highway linking State Highway 1 at Manukau with State Highway 16 in Point Chevalier, via Māngere and Onehunga. Along with its spurs, State Highway 20A and 20B, the state highway serves Auckland Airport, the country's largest, therefore making SH 20 a key arterial route connecting the airport to the wider Auckland region and most of the upper North Island. The route also forms the southern part of the Western Ring Route, a 48 kilometres (30 mi) motorway route bypassing central Auckland.

Quick Facts Route information, Length ...

State Highway 20
Southwestern Motorway
The Southwestern Motorway seen from Puketāpapa / Mount Roskill in 2011
Route information
Maintained by NZ Transport Agency
Length	25 km (16 mi)
Existed	1977–present
Major junctions
South end	SH 1 (Auckland Southern Motorway) at Manukau City Centre
Major intersections	SH 20B (Puhinui Road) at Puhinui SH 20A (George Bolt Memorial Drive) at Māngere
North end	SH 16 (Northwestern Motorway) at Waterview
Location
Country	New Zealand
Highway system
New Zealand state highways Motorways and expressways List
← SH 18 → SH 21

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SH 20 begins from the Auckland Southern Motorway (SH 1) at Manukau, then heads northwest to Māngere. It then runs over the Māngere Bridge to Onehunga, whereupon it continues northwest, cutting through Mount Roskill and Owairaka. It then enters the Waterview Tunnel, New Zealand's longest road tunnel,^[1] before ending at a junction with the Northwestern Motorway (SH 16)

A motorway linking the Northwestern and Southern sections of Auckland was recognised early in the planning stages for the Auckland motorway system, and land along the Southwestern Corridor was protected against development in the 1950s.^[4] The Southwestern Motorway was a prominent aspect of a 1963 master transport plan for Auckland created by the US consultancy firm De Leuw Cather and Co, which identified a need for a motorway linking the Northwestern Motorway, Dominion Road, Māngere and Wiri, also identifying the need for a rapid transport rail route along the same corridor.^[4] In 1968, the Auckland Regional Authority began planning agreements for the route between the Northwestern Motorway and Dominion Rd, while the National Roads Board (now a part of the NZ Transport Agency) began organising the route southeast, between Dominion Road and Wiri.^[4]

The first section of what would become the Southwestern Motorway was the Onehunga Bypass from Queenstown Road in Hillsborough to Neilson Street in Onehunga in 1977, which terminated at the Old Mangere Bridge, a low-lying concrete structure built in 1915.^[5] The contract for a new motorway bridge over the Māngere Inlet between Onehunga and Māngere Bridge was let in July 1974, with work expected to take three years to complete.^[6] In May 1978 the construction halted when workers organised a labour strike over insufficient redundancy payments. The partially constructed bridge was picketed for a period of two and a half years, becoming the longest continuous labour strike in the history of New Zealand.^[7] Eventually, the bridge was opened in February 1983,^[6] and by 1984 the motorway extended south to Coronation Road.

The construction of the motorway had a strong impact on the Te Puea Memorial Marae community in the Māngere Bridge suburb, as the motorway cut off pedestrian access to the surrounding areas. Land reclamation as a part of the bridge construction meant the marae was no longer adjacent to the Māngere Inlet, and the land surrounding the marae was redesignated as a light industrial area.^{[8][9][10][11]}

The Auckland Comprehensive Transportation Study Review (1986) confirmed the Southwestern Motorway between Dominion Road and Māngere was a top priority of the Auckland Regional Authority.^[4] By 1987, the Papatoetoe Bypass was completed, linking Massey Road to the south, ending at a roundabout meeting Puhinui Road.^[5] The gap between Coronation Road and Massey Road, known as the Māngere Extension, was not completed until 1997.^[5] The Manukau Extension, connecting Puhinui Road to the Southern Motorway (bypassing Wiri Station Road) was completed in 2010.^[12][5] A duplication of the Māngere Bridge, with future provision for rail, was also opened in 2010.^[13][14]

The northern section was first extended to Hillsborough Road in 1989.^[15] Work began on the Mt Roskill Extension in August 2005, with construction of the section ending at Maioro Street/Sandringham Road finishing on 15 May 2009.^[15][5] The final section of the Southwestern Motorway, the Waterview Connection, connecting Maioro Street with SH 16 at the Great North Road Interchange commenced construction in 2012 and opened to traffic in July 2017.^[1][16]

• List of New Zealand state highways
• List of motorways and expressways in New Zealand