A section of the M1 could get a 60mph speed limit during rush hour under new plans to reduce pollution.
Highways England is considering the initiative, which would come into force along a stretch of motorway that runs close to schools and homes in Sheffield.
The motorway between junction 28 and 35a already breaches UK and EU air quality limits hundreds of times each year, but it's set to get worse as the Department for Transport is converting it to a smart motorway.
Under the £106 million scheme, between 5,000 and 10,000 more cars are expected to travel the route because the hard shoulder will be opened during peak periods.
However, Highways England's own reports suggest that increasing traffic flow to that degree could make nearby residents' exposure to pollution hit illegal levels.
The organisation's operations director Nick Harris wrote to local MPs last month, saying: "We are looking into initiatives that might avoid or reduce the need for these speed limits."
One such initiative, which involved painting barriers with special paints that soak up pollutants, had already been deemed unsuccessful.
An AA spokesman said that motorists were being unfairly targeted by schemes like this.
He said: "Car users are always the easy hit when it comes to pollution when actually they are not one of the main contributors.
"There is a very good chance that the traffic is already moving at that speed during rush-hour."