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The underpass is built within part of the former Kingsway tramway subway, which was 20 ft (6.6 m) wide and allowed bi-directional flow because of the fixed rails and relatively narrow width of the trams. The new underpass was built by John Mowlem & Co and opened on 21 January 1964. It is only 17 ft (6.25 m) wide, and consequently normally one-way northbound because of the side clearances required. The headroom is only 12 ft 6 in (3.75 m) due to the tunnel having to pass beneath the bridge abutment by a 1:12 gradient. An electronic 'eye' alerts drivers of tall vehicles and diverts them to an 'escape route' to the left of the entrance. However, high vehicles do still try to go through and so get stuck occasionally.
The underpass is a concrete box within the former tram subway, with the road surface at the original track level. At the northern end of the underpass the road rises to the surface on a new carriageway supported by metal pillars. This passes through the site of the former Aldwych tramway station: because of the greater width requirement, 27 trees and some pavement were removed for it to be constructed.
The tunnel is used by the 521 bus route northbound. In 2012, the direction of traffic in the tunnel was temporarily reversed, so that it was in use by southbound traffic. This was to facilitate easier traffic flow during the 2012 Summer Olympics.
- Mowlem 1822 - 1972, p.6
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