Plans to reduce the speed limit to 60mph on another stretch of motorway have been announced.
The latest proposal, from the Highways Agency (HA), affects a three-mile section of the M3 between junction 3 at Lightwater in Surrey and junction 4 near Farnborough.
This section is part of a 13.4-mile stretch that is being upgraded in a £174 million scheme, with the 60mph proposal, to operate between 7am and 7pm daily so air quality levels can be met, now going out for consultation.
Last month the HA announced a consultation, also on air quality grounds, on introducing a 60mph speed limit for a 32-mile stretch of the M1 from junction 28, near Matlock in Derbyshire, to junction 35a, north of Rotherham in South Yorkshire.
On the M3 scheme, the HA said the upgrade, with extra lanes being added, would ''see motorists have more reliable and free-flowing journeys and save time going in and out of London''.
Roads Minister Robert Goodwill said: ''The M3 is heavily congested and the introduction of this smart motorway will help improve journey times by smoothing traffic flow.''
The agency said improvements in vehicle technology were expected to remove the need for any speed restriction due to air quality on this part of the M3 within four years.
But motoring organisations questioned the plans.
AA president Edmund King said: ''The AA believes that the extra capacity is required and that ironically 60mph speeds at peak times would actually speed up journeys.
''However a majority of drivers question whether reducing motorway speeds will make any difference to air pollution as 69% see lorries as the main culprit.''
RAC technical director David Bizley said: ''Coming hot on the heels of the M1 announcement, the M3 plan invites the question as to whether any of the eight other planned all-lane running, smart motorway schemes which the Government have heavily invested in will also need to have reduced speed limits put in place to protect air quality.''