The interchange between the M25 and the A3 near Wisley in Surrey is to be redesigned to create four dedicated link roads for all drivers making left turns at the junction. Drivers turning right will use a new enlarged junction roundabout.
The A3 will be widened from three lanes to four between Ockham and Painshill in both directions, with two lanes remaining over the M25.
Construction on the £175m scheme is scheduled to begin in 2020.
The plans have been firmed up after a public consultation process at the start of the year. The selected proposals were referred to as Option 14 during the consultation. The other shortlisted proposal, known as Option 9, was a huge four-level flyover with dedicated free-flow slip roads for traffic accessing the M25 from the A3. This £215m option was rejected due to concerns about environmental impact.
The junction improvement project also includes improved routes for pedestrians and cyclists, a green bridge linking Wisley and Ockham Commons, and better access to RHS Garden Wisley via a new bridge and link road to the east of the A3. The proposals are designed tominimise the impact on trees within RHS Garden Wisley and ancient woodland near Ockham.
The M25/A3 interchange is a key congestion pinch point on the strategic road network and has one of the highest recorded collision rates across the Highways England network.