The first tunnel under the River Mersey was for the Mersey Railway in 1886. The possibility of a Mersey road crossing was first discussed in the 1890s. During the 1920s there were concerns about the long queues of cars and lorries at the Mersey Ferry terminal so once Royal Assent to a Parliamentary Bill was received construction of the first Mersey Road Tunnel started in 1925, to a design by consulting engineer Sir Basil Mott. Mott supervised the construction in association with John Brodie, who, as City Engineer of Liverpool, had co-ordinated the feasibility studies made by consultant Engineers Mott, Hay and Anderson. The main contractor was Edmund Nuttall. In 1928 the two pilot tunnels met to within less than 25 millimetres (1.0 in).

The tunnel entrances, toll booths and ventilation building exteriors were designed by architect Herbert James Rowse, who is frequently but incorrectly credited with the whole civil engineering project. These are Grade II listed buildings. More than 1.2 million tons of rock, gravel, and clay were excavated; some of it used to build Otterspool Promenade. Of the 1,700 men who worked on the tunnel during the nine years of its construction, 17 were killed.

At the time of its construction it was the longest sub-aqueous tunnel in the world, a title it held for 24 years. The tunnel, which cost a total of £8 million, was opened on 18 July 1934 by King George V; the opening ceremony was watched by 200,000 people.[citation needed]
By the 1960s, traffic volume had increased. In 1971 the Kingsway Tunnel opened to relieve congestion.

The tunnel is 3.24 kilometres (2.01 mi) long. It contains a single carriageway of four lanes, two in each direction. Different height restrictions apply to the nearside and offside lanes in each direction, because of the curvature of the tunnel. These are 3.9 metres (13 ft) and 4.75 metres (15.6 ft) respectively, and there is a 3.5t weight limit for goods vehicles. All buses are required to use the offside lane, regardless of their height. Lane signals (consisting of an illuminated green arrow or red cross) are displayed at regular intervals, although under normal circumstances none of the lanes are currently used bidirectionally. This is in contrast to the Kingsway Tunnel, where lanes in toll concourse are alternated to prioritise higher traffic in one direction during peak hours.

The tunnel has two branches leading off the main tunnel to the dock areas on both sides of the river. The Birkenhead branch tunnel (known as the Rendel Street branch) was closed in 1965. The Liverpool branch tunnel remains in use, in the exit direction only. It emerges opposite the Liver Building, next to the Atlantic Tower Hotel and Church of Our Lady and Saint Nicholas. Originally, it carried two-way traffic and the junction inside the tunnel was controlled by traffic lights, but this arrangement was discontinued to reduce the delays brought on by increasing traffic levels.
[edit]Today

When driving through the tunnel, it appears as semi-circular. It is circular, however, and the area below the roadway is known as Central Avenue. The roadway acts freestanding. The area beneath the roadway was planned to house an electric tram route, but it was instead used to house a gas pipe which was later abandoned.

In April 2004 construction began of seven emergency refuges below the road deck, each capable of holding 180 people, as part of a £9 million project to bring the tunnel into line with the highest European safety standards. Each refuge is 21 metres (69 ft) long and 3 metres (9.8 ft) wide, accessible from the main tunnel walls. The refuges have fire resistant doors, ramps for wheelchair access, a supply of bottled water, a toilet, and a video link to the Mersey Tunnels Police control room. All seven refuges are linked by a walkway below the road surface, with exits at the Liverpool and Birkenhead ends.