A new 60mph speed limit on a section of the M1 is being proposed by the Government in a bid to cut air pollution.
The reduced speed limit would apply to a 32-mile stretch between junction 28, near Matlock in Derbyshire, and junction 35a, north of Rotherham.
While variable speed limits have been widely used to help traffic flow, the RAC points out this could be the first time they have been used to cut air pollution.
The proposal has only been put out to consultation at this stage. Nevertheless as it announced the plan, the Highways Agency said the 60mph limit would most likely remain in place for "several years" and it could "pave the way for similar restrictions on other sections of motorway".
As well as cutting air pollution it said the move will bring "reduced congestion, increased capacity and improved journey time reliability for users of the motorway" but it conceded there will be a "negative impact on business efficiency and individual mobility".
There would continue to be a normal 70mph speed limit on the rest of the M1, which runs between north London and Leeds.
RAC technical director David Bizley 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 34-mile 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 to reduce congestion.
"More worryingly, 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.
"This very powerfully demonstrates the impact that speed has on emissions and many will be surprised to hear that a reduction of just 10mph can have such a significant effect on improving air quality."
He added: "Perhaps the Government should be considering reintroduction of incentives to scrap older high polluting vehicles to minimise the need for speed restrictions of the type proposed."
Copyright Press Association 2014