A proposal to implement a speed limit of 60mph on a stretch of the M3 is being considered by the Highways Agency.Rush-hour congestion slows traffic on the busy route into London to an average of 45mph in the morning and when leaving the city to an average of 56mph in the evening. The planned £174m upgrade will transform a 13.4 mile stretch of the route into a ‘smart motorway’ by adding an additional lane in both directions between junctions 2 and 4a and using variable speed limits to manage traffic flow.According to the HA, these measures will increase capacity, reduce congestion and boost average speeds for motorists to cut four minutes per vehicle from a morning commute into London and one minute per vehicle from the evening journey.Due to the capacity increase, emissions from the extra traffic are forecast to breach local air quality limits on a short section of the route unless the section is limited to 60mph during certain times of the day.The Highways Agency is consulting on proposals to introduce a 60mph speed limit on the three mile section from junction 3 at Lightwater to junction 4 near Farnborough between 7am and 7pm. The changes will bring air quality levels within the permitted legal limit and due to additional lane capacity - motorists should experience higher speeds than they do today at peak times.Proposals to limit speed due to air quality issues are limited to just 38 miles of the entire 4,300-mile strategic road network – less than one per cent. Improvements in vehicle technology are expected to remove the need for any speed restriction due to air quality on this part of the M3 within four years.Roads Minister Robert Goodwill (pictured) 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.“Introducing speed controls for air quality reasons is proposed for less than one per cent of the strategic road network. Peak time speed limits on the M3 are currently below 70mph. However let’s be clear, no decisions have been taken yet but the risks need to be managed responsibly, with any proposed limit removed as soon as air quality levels improve.”The eight-week Highways Agency consultation is required as the changes will need new regulations.