A trial to allow cars to use the hard shoulder to ease congestion is to be introduced on a West Midlands motorway.
Road campaigners say the plan could risk drivers' lives
Drivers on the M42 will be directed to use the hard shoulder during peak hours via overhead signs.
If traffic operators spot a broken down vehicle in the hard shoulder along the 11-mile stretch the lane can be closed down, again using overhead signs.
The Highways Agency said the scheme aims to ease congestion but safety groups have warned it could risk lives.
The pilot will begin between junction 3a (for the M40) and junction 7 (for the M6) of the M42, southwest of Birmingham.
A Highways Agency spokesman said it could be an alternative to building extra motorway lanes and could be rolled out across the country if the pilot is successful.
"Motorway widening is very expensive and bad for the environment.
"This is a trial looking at how we can make the best use of the space we already have available."
The scheme will allow motorway operators to open and close lanes, including the hard shoulder, and is due to begin on 12 September.
A red cross above the hard shoulder will indicate it is closed while 50mph sign, the speed limit when the hard shoulder is open, will indicate the lane is in use.
Signs on gantries can be changed within seconds if operators spot a problem in the lane. New lay-bys have been built along the stretch for broken-down cars to pull into.
But national road safety charity Brake warned it could be a "potentially life-threatening alternative".
Its director Mary Williams said: "Measures taken to ease congestion should not put safety at risk.
"The hard shoulder serves a vital purpose on a motorway to enable emergency services to quickly reach road crashes and respond as soon as possible to those injured or possibly dying."
'Change the system'
Concerns were also raised by the Royal Society for the Prevention of Accidents.
Its head of road safety, Kevin Clinton, said: "Using the hard shoulder as a running lane may make it more difficult for drivers to find somewhere safe to stop if they break down, as the emergency refuges are only spaced at intervals along the motorway.
"Emergency service and breakdown vehicles may also find it more difficult to reach breakdowns and accidents, which would delay accident victims receiving help and delay the motorway being cleared.
"The Highways Agency will need to monitor the trial very carefully to see whether these turn out to be problems in reality and if so, be prepared to change the system."