Upgrade work at M25 junction 10, where the motorway crosses the A3. The scheme is intended to relieve the chronic congestion suffered at this busy junction. At present all turning traffic passes through one large signalised roundabout which is operating beyond the limits of its capacity for significant parts of the day.
A large number of options were considered for the work, including radical plans to replace the junction entirely with a new free-flowing four-way interchange of one type or another. However, there are now two options remaining which have gone out to consultation.
- Option 9, described as a "four-level flyover", would add a new flyover at a fourth level over the top of the existing junction. This would carry right-turning traffic from the A3 southbound to the M25 clockwise and from the A3 northbound to the M25 anticlockwise. New left turns from the A3 in each direction would also be built. Traffic exiting the M25 would continue to use the roundabout as at present, but the reduced volume of traffic remaining on the junction would mean that this traffic could be accommodated without significant delays.
- Option 14, which would see the existing roundabout elongated to the east and west with new bridges built across the M25. The new roundabout would have more lanes and would continue to be signalised as now.
Option 9 is effectively half of a four level stack, and allows the possibility that at a future date the remaining sliproads could be added, perhaps at the level of the existing roundabout, in order to make the junction fully free-flowing.
Both options also involve widening the A3 to provide a fourth lane in both directions to the north and south of the junction. The road widening effectively does not form part of the consultation, as it appears in all possible versions of the scheme, but appears to be the most controversial element of the proposals. Widening the A3 to the south of the junction may involve clearing some 500 trees in the grounds of the Royal Horticultural Society's gardens at Wisley, some of them rare or unusual species, and one of which was planted by the Queen to mark her Silver Jubilee in 1977.
Option 14, the enlarged roundabout, has been selected as the preferred route, although with slight modifications to remove the traffic lights on the left turn lanes.