60mph speed limit proposal for stretch of M1 motorway
The government is proposing to set up a 60mph speed limit for a 32-mile stretch of the M1, in a bid to cut pollution.
The Highways Agency says the new restriction would apply from 7am to 7pm, seven days a week.
It would be in place from junction 28, near Matlock, Derbyshire, to junction 35a, north of Rotherham.
Variable speed limits are widely used to aid traffic flow, but the RAC said this could be the first time they had been implemented to cut air pollution.
It warned that reducing the maximum speed from the national standard 70mph to 60mph could "pave the way for similar restrictions on other sections of motorway" and there would "inevitably be a negative impact on business efficiency and individual mobility".
The Highways Agency said the lower limit, which has been put out to consultation, was likely to remain for "several years".
In its document, it stated that the current use of the 70mph speed limit for motorways was having "adverse impacts on air quality" and that cutting it would reduce emissions.
It also said the change would bring "reduced congestion, increased capacity and improved journey time reliability for users of the motorway".
The normal speed limit would still apply to the rest of the M1, which runs from north London to Leeds.
Tougher European Union guidelines on air quality have come into force and the agency argued that a lower limit would be needed to ensure these were met on the section from junctions 28 and 35a, which goes past Mansfield, Chesterfield and Sheffield.
It said: "For the purposes of this consultation, it should be assumed that the speed limit will need to be in place for several years. However, we are not able to give an indication in this document of how many years the speed limit will need to be retained."
But the document said: "It is expected that vehicle emissions will reduce as more new, cleaner vehicles come into use and older, more polluting vehicles become obsolete."
The agency said it could change its plans, by limiting the operation of the lower speed limits to peak hours, or Mondays to Fridays.
The length of the stretch of road affected could be shortened too, it added.
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."
He added that 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".
Mr Bizley also said: "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."
The consultation will close on 3 March.