Campaigners fear speed limits across the country could be cut if plans for a 60mph limit on the M1 in South Yorkshire go ahead.
The Highways Agency wants the new limit to operate on a 30-mile stretch of the M1 past Sheffield – between Junction 28 near Alfreton and Junction 35a at High Green – from 7am to 7pm.
It claims it would help reduce pollution.
However, the RAC said the move could pave the way for similar speed restrictions elsewhere.
The plan is part of the ‘smart motorway’ scheme, with the hard shoulder used as an extra lane.
David Bizley, RAC technical director, said: “This is a landmark proposal as to the best of our knowledge motorway speed limits have not previously been lowered in order to comply with environmental legislation.
“If this becomes reality for the stretch of the M1, which seems highly likely, it would certainly negate some of the current benefits of operating this section as a ‘smart’ motorway where motorists are allowed to use the hard shoulder.
“It could pave the way for similar restrictions on other sections of motorway.
“While preserving air quality is obviously a paramount concern there will inevitably be a negative impact on business efficiency and individual mobility.”