Highways England estimates the cost to be between £335.5m and £478.9m
Plans to convert the M25 between the A3 and M40 junctions into a “smart” motorway are in the preliminary design stage.
Highways England, which manages the smart motorway projects, says the designs should be completed by late spring.
The agency estimates the cost of the project to be between £335.5 million and £478.9 million, with a start date of 2020/21.
It says the section between Junction 10 A3 and Junction 16 M40 has two-way traffic flows of more than 200,000 vehicles a day, the highest on its strategic road network. The amount of traffic results in “frequent congestion and severe delays” for road users.
Highways England says: “The government announced that its preferred solution was for a smart motorway with all-lane running at junctions 15 M4 to 16 M40 and also at junctions 10 A3, 11 Chertsey and 12 M3.
“This means the existing hard shoulder would be converted into a traffic lane providing five lanes for traffic at junctions 15 to 16 and four lanes elsewhere.
“Regular refuge areas are available in place of the hard shoulder for emergency use.”
As part of plans for the new third runway at Heathrow the airport proposes to build a tunnel for the M25 between junctions 14 Heathrow and 15 M4.
Heathrow aims to move the carriageway 150 metres west of its current site and lower it seven metres into a tunnel.
A spokesman for Highways England said officers are working with the airport and other schemes in the M25 south west quadrant.
He said: “The M25 smart motorway design activities started before the government made its decision on Heathrow so we are aware that a small part of the smart motorway design, around Junction 14, may become redundant depending on the future design of Heathrow.
“The remainder of the smart motorway scope remains unchanged by the airport expansion.”
Highways England is currently preparing a planning application for the redevelopment of the M25 junction with the A3. It says it will submit a development consent order to the planning inspectorate in “early 2019”. If granted work will start in 2020/21.
Highways England has already converted the M3 between Junction 2 M25 and Junction 4a Farnborough into a smart motorway. The project took more than two years to complete and cost £174 million.
It has attracted criticism from motorway users. Paul Mayhew, from Walton, was one of many people to contact Surrey Live. He said: “No hard shoulder at all is potentially lethal, and the overhead signage is badly utilised and managed… drivers have little faith in it.”
The M23 is being currently being converted into a smart motorway, between junctions 8 M25 and 10 Crawley.
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