The Strand Underpass is a vehicle tunnel

Tunnel

A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...

 in central London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 connecting Waterloo Bridge

Waterloo Bridge

Waterloo Bridge is a road and foot traffic bridge crossing the River Thames in London, England between Blackfriars Bridge and Hungerford Bridge. The name of the bridge is in memory of the British victory at the Battle of Waterloo in 1815...

 to Kingsway

Kingsway

-Canada:*Burnaby—Kingsway, a federal electoral district in British Columbia between 1988 and 1997*Kingsway , a road in Edmonton, Alberta *Kingsway , a road in Vancouver, British Columbia...

 near Holborn

Holborn

Holborn is an area of Central London. Holborn is also the name of the area's principal east-west street, running as High Holborn from St Giles's High Street to Gray's Inn Road and then on to Holborn Viaduct...

. It was built by John Mowlem & Co

Mowlem

Mowlem was one of the largest construction and civil engineering companies in the United Kingdom. Carillion bought the firm in 2006.-History:Founded by John Mowlem in 1822, the company was awarded a Royal Warrant in 1902 and went public on the London Stock Exchange in 1924. It acquired SGB Group in...

 and opened on 21 January 1964.

It is built within part of the former Kingsway tramway subway

Kingsway tramway subway

The Kingsway Tramway Subway is a cut-and-cover Grade II Listed tunnel in central London, built by the London County Council, the only one of its kind in Britain...

. The original tramway tunnel was 20 ft (6.6 m) wide and allowed bi-directional flow because of the fixed rails and relatively narrow width of the trams; however, the new road tunnel was only 17 ft (6.25 m) wide, and consequently only northbound traffic was permitted, 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

Abutment

An abutment is, generally, the point where two structures or objects meet. This word comes from the verb abut, which means adjoin or having common boundary. An abutment is an engineering term that describes a structure located at the ends of a bridge, where the bridge slab adjoins the approaching...

 by means of a 1:12 gradient

Gradient

In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....

. 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.

Underground, the new underpass is constructed in a concrete box within the former tram tunnel; the road surface, however, is at the original track level. At the northern end of the tunnel, 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

Aldwych tramway station

Aldwych tramway station was a tram stop underneath Kingsway, a road in central London. It was built in 1906 by the London County Council Tramways as part of the Kingsway tramway subway, a subterranean tram route which was constructed to join the separate networks of tramways in North and South...

, but because of the greater width requirement, 27 trees and some pavement had to be removed for it to be constructed.

The tunnel is used by the 521 bus route

London Buses route 521

London Buses route 521 is a Transport for London contracted bus route in London, United Kingdom. The service is currently contracted to Go-Ahead London.-History:Service 521 is one of two Red Arrow routes, the other being route 507, that survived into 2009....

on Northbound journeys.