Potential rises to speed limits have been criticised by road safety charities after Liz Truss hinted that restrictions could be left up to the judgement of drivers.

Her comments made in Wednesday’s Tory leadership hustings in London came as she also pledged to ban the failed “experiment” of smart motorways.

She said that she would be “prepared to look at” speed limit changes, after being asked what she would do to stop long delays that bring about “lower than necessary speed limits” on smart motorways.

Last night charities warned that increases could create unsafe journeys and lead to more accidents, and that they would oppose them.

BIRMINGHAM, ENGLAND - AUGUST 23: Foreign Secretary and Conservative leadership hopeful Liz Truss speaks on stage on August 23, 2022 in Birmingham, England. Foreign Secretary, Liz Truss and former Chancellor Rishi Sunak are vying to become the new leader of the Conservative Party and the UK's next Prime Minister. (Photo by Anthony Devlin/Getty Images)

“It's a reality that people can and do continue to make mistakes behind the wheel - and the faster we drive, the greater our risk of collision and the harder the impact,” said Mary Williams, CEO of Brake, the road safety charity.

“Any change to speed restrictions on motorways must be evidence based, taking into account road user behaviour, road infrastructure and vehicle design to ensure safe roads for all – it is highly unlikely that the evidence base for motorway safety right now would point to a recommendation for higher speed limits, and Brake would not therefore support such a recommendation."

Becky Guy, Road Safety Manager at The Royal Society for the Prevention of Accidents said that all the current evidence pointed to the fact that slower journeys were safer ones.

“Driving at the national speed limit in all motorway situations doesn’t actually guarantee a faster or smoother trip,” she said.

“Studies have shown that lower motorway limits help maintain smoother traffic flows, reduce air pollution and save drivers fuel. And slower is always safer.

“When variable speed limits were applied to a section of the M25 in 1995 a 15 per cent drop in injury crashes was observed, and similar results have been observed across the world.”

The proposal to scrap smart motorways was much more favourably received after a long campaign from charities and families who have lost loved ones during them.

Campaigners have argued that these are dangerous because of a lack of hard shoulders for cars to pull into when they break down, leading to avoidable deaths on the roads.

Grant Schapps, the Transport Secretary earlier this year announced a moratorium on new smart motorways until five years of safety data can be collected.

Answering questions at London’s Tory hustings, she said “I do believe that the smart motorways experiment hasn't worked”, firmly answering “yes” when asked if she would scrap them.

It comes after Rishi Sunak said last month that he would ban new smart motorways if he became prime minister

Earlier this year South Yorkshire Police decided that Highways England bosses should not face corporate manslaughter charges after deaths following two collisions on the M1.

Nargis Begum, 62, died in 2018 after her car was hit by another vehicle on part of the motorway with no hard shoulder.

Another collision in 2019 led to the deaths of Jason Jason Mercer and Alexandru Murgeanu în June 2019, where a lack of a hard shoulder was deemed to have contributed.

“We’re supporting the moratorium on the roll-out of new smart motorways until more safety evidence is available,” said Ms Guy.

Road safety charities criticise changes to speed limits mentioned by Liz Truss