The Highways Agency is considering whether to cut the daytime speed limit on a section of the M3 to 60mph.
It today began consulting on the idea to reduce the maximum legal speed to 60mph on a 4.6km long section of the motorway between junction 3 at Lightwater and junction 4 near Farnborough between 7am and 7pm.
The speed restriction would accompany the £174M widening of the M3 in Surrey, which will add an extra lane in both directions between junctions 2 and 4a.
The capacity increase means traffic emissions could breach air quality limits unless the proposed speed limit is reduced.
“By upgrading more than 20.9km of the M3 to a smart motorway and adding an additional lane in both directions, we expect to see motorists have more reliable and free-flowing journeys and save time going in and out of London,” said Highways Agency major projects group leader Sue Housley.
“To protect air quality on the 4.6km section of this scheme we are proposing that it operates at 60mph at certain times of the day. This is a solution for a very small section of the M3.”
The Agency last month began consulting on a reduced speed limit for part of the M1 between junctions 28 and 35a; and completed consultation on the same change on the A556 trunk road.
The Agency’s proposals for limiting speed to mitigate air pollution on 61km of the 6,900km strategic road network.
It hopes improvements in vehicle technology will remove the need for such measures in future.
The eight-week M3 consultation is required as imposinpg the new speed limit would require new regulations.
The Agency said congestion already slowed traffic on the route into London to an average of 45mph in the morning and to 56mph on the way out of the capital in the evening.
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. Variable speed limits on our motorways are nothing new and have been used for many years to cut congestion and keep vehicles moving.”