Plans to use the hard shoulder on 31 miles (50km) of the M4 motorway to reduce congestion have been met with caution by the AA and haulage firms.
Junctions 3 to 12 between Heathrow Airport and Theale, Berkshire, would become a managed motorway under plans announced by the government.
The AA fears the plans would leave motorists in a dangerous situation in case of a breakdown.
The Highways Agency said interactive signs would ensure safety.
The system was first used on the M42 in the West Midlands. It is also currently used on sections of the M25 and M1.
The Highways Agency estimates the M4 scheme will cost £525m to £720m.
A managed motorway means, at peak times, the hard shoulder can be opened up as an additional traffic lane and variable speed limits introduced.
Andrew Page-Dove, of the Highways Agency, said: "We've learnt a lot from the M42 scheme where the hard shoulder was open to traffic at peak times and then closed at other times.
"People find it quite confusing, using the hard shoulder when they shouldn't. That creates additional dangers for road workers and people who have stopped legitimately.
"We're trying to introduce something a lot more intuitive, like we use on sections of the M25.
"There won't be a hard shoulder, but all the control mechanisms will be there."
Paul Watters, of the AA, said: "For people who break down, the hard shoulder is a safe haven.
"To take it away permanently, when congestion only exists for a few hours each day, would create an undue risk."
Steve Bowles, of haulier Roy Bowles Transport, based in Colnbrook, Berkshire, believes the scheme will do little to help.
He said: "I think it will endanger road safety.
"I've used the M42 during rush-hours and even though the hard shoulder is only used then, it's still not safe.
"You always need a place to pull over if your vehicle breaks down on the motorway, especially a heavy goods vehicle."