A 20mph speed limit is now in force on nearly one third of the country’s roads, with campaigners pushing for it to become the default across the nation.

Over 50 per cent of London’s roads are already restricted to a 20mph limit, and supporters claim adopting the lower speed more widely will cut road deaths and serious injuries.

The calls for a reduction come after trials by the Welsh government that saw eight 20mph pilot schemes rolled out in built-up areas ahead of it becoming the national limit in April 2023.

More than 21 million people in the UK currently live or will live in areas that have a 20mph limit after Cornwall and Cambridgeshire voted to expand 20mph schemes in the May elections.

The United Nations is also promoting 20mph limits as the international default for streets "where people walk, live and play".

Richard Cuerden, a director of TRL, formerly the Government’s Transport Research Laboratory, called the current guidance and regulations completely "out of date". Rule 124 of the Highway Code says that if there are street lights and no signs to the contrary, the speed limit is 30mph.

Mr Cuerden told the Sunday Times: "We need to be thinking about 20mph. Until recently we talked about traffic accidents and accepted that people were killed or injured on our roads as a consequence of going about their daily life. It is not acceptable to us any more."

Critics have said reducing the speed limit to 20mph for motorists is "nonsensical" and that there is no evidence to suggest it will reduce cases of serious accidents.

Brian Gregory, the policy director and founding member of the Alliance of British Drivers, said the lower speed limit may in fact reduce road safety.

He told The Telegraph: "If you are driving at 20mph, you are focusing on the speedometer and not looking out of the window. There is evidence to suggest that when you focus on the speedometer your peripheral vision gets worse, which is what you need if you are trying to avoid accidents.

"It’s completely nonsensical because the restriction does not work on any level when you think about it from a common sense perspective."

Risk-taking behaviour from motorists may increase due to a false perception that 20mph is not a dangerous speed to drive at, he added. 

The 20mph campaigners estimates that by bringing in the lower limit casualties would fall by about one sixth, and taxpayers would be saved £11.8 million a year.

Rod King, founder of the 20’s Plenty For Us campaign group, told the paper that the cost of making extensive changes to signs is one of the main factors holding authorities back from bringing in the limit more widely.

In the borough of Westminster, it would reportedly cost £1 million to implement 20mph limits on most roads, according to analysis of Government figures carried out by the campaign group.

Sources at the Department for Transport have said there are no plans to review the national speed limits for England, pointing to the simplicity of the current system.