Ministers say the policy will be "paused" - until five years of safety data for such schemes introduced before 2020 has been collected.

The proposals were revealed by Highways England amid some fanfare in 2019, despite criticism from experts who claimed they could endanger drivers.

Here, Sky News explains the key issues surrounding smart motorways.

What are smart motorways?

Smart motorway is a term applied to sections of motorway in the UK that use various forms of technology and active traffic management (ATM) techniques to increase capacity.

They were considered desirable by a series of governments as a quicker and more cost-effective alternative to widening carriages.

First introduced on 14 miles of the M25 in 1995, it was expanded to cover hundreds more miles of motorway across the country from the 2000s onwards.

While they are primarily used in England, they are also known in Scotland as intelligent transport systems.

Smart motorways fall into three main categories:

• Controlled: Motorways that use variable speed limits, but which have permanent hard shoulders.
• Dynamic: Sections of motorway where the hard shoulder is selectively open to moving vehicles when traffic levels are deemed to be too high for three lanes. The speed limit is reduced to 60mph when this happens.
• All-lane running: Where variable speed limits are employed, but there is no hard shoulder and all lanes are permanently open to moving vehicles.

The last of these is the type covered by the latest government announcement.

On these stretches of motorway, the hard shoulder is only closed to traffic in the event of an incident - in which cases a red X will appear above the lane on an overhead gantry.

They feature more regular emergency refuse areas for drivers who break down or have an accident than on standard motorway.

Where are the country's smart motorways?

The network of smart motorways has been developed along some of the country's busiest stretches of high-speed road.

The traffic management technique, including hard-shoulder running, was first used in the UK on a part of the M42 in the West Midlands in 2006.

Now covering around 375 miles, smart motorway sections have since been introduced on sections of the M1, M3, M4, M5, M6, M20, M23, M25, M27, the M40/M42 interchange, M56 and M62.

How much have they cost?

It was estimated in 2007 that ATM could be rolled out at a cost of £5-15m per mile, compared to the £79m per mile carriage widening was deemed to cost.

A total of £2bn worth of contracts to extend the schemes were announced by the Conservative government in 2010.

It was estimated in November 2020 that the planned rollout of smart motorway would cost taxpayers another £1.2bn.

Although the total expenditure on smart motorways since their introduction is difficult to calculate, the Department for Transport is committing £900m to improve safety on the existing network over the next four years.

What do critics say about smart motorways?

Criticism of smart motorways has straddled a number of issues.

Environmental experts say they do nothing to reduce traffic or harmful carbon dioxide emissions, while Friends of the Earth has previously suggested they could actually increase emissions.

However, the most intense condemnation of the schemes has surrounded safety issues.

A Freedom of Information request in 2020 revealed 38 people had been killed on smart motorways in the previous five years.

It also showed the number of "near misses" had increased on one section of motorway by 2,000% to 1,485 in the five years since it had been converted.

The RAC has voiced concerns over the risk to motorists presented by smart motorways, while the AA has said they "compromise road safety".

A report from Commons Transport Select Committee (TSC) in November said plans to remove the hard shoulder from all future smart motorways and use the lane for live traffic were "premature" and recommended they were suspended.

What is happening with smart motorways now?

As well as halting the rollout of new all-lane-running smart motorways until five years of data has been collected, the government has pledged to "revisit the case" for installing controlled smart motorways instead.

Transport Secretary Grant Shapps said: "While our initial data shows that smart motorways are among the safest roads in the UK, it's crucial that we go further to ensure people feel safer using them.

"Pausing schemes yet to start construction and making multimillion-pound improvements to existing schemes will give drivers confidence and provide the data we need to inform our next steps."