Where was the project located?
Work took place on the northwest section of the M25 near Watford, between junctions 16 and 23.

Why was the scheme needed?
The M25 is one of Europe's busiest motorways, with some sections handling around 200,000 vehicles every day. It is at the core of our network and suffered from increasing congestion levels and journey times. Traffic moves slowly, especially at busy times of the day.

*The estimated final cost for this scheme is £594 million. This figure is subject to final account closures and may result in a variation of this figure.

What work was carried out?
We carried out construction works on the Berry Lane Viaduct to allow widening of the M25 to 4 lanes in each direction. Site clearance included thinning out young trees adjacent to the viaduct. Mature trees at the edge of the working area were retained where it was safe to do so. Re-planting took place at the viaduct abutments.

We then widened the carriageways from 3 lanes to 4 lanes in each direction. This work was undertaken in phases. We have made use of the existing hard shoulders for the new lanes and created new hard shoulders alongside. We have developed a design to do this within land we already own. We maintained the existing numbers of traffic lanes in each direction during busy times to avoid significant traffic disruption caused by the works. At quieter times, some lanes were closed. This helped us to work as quickly as possible.

You can view archived material for this project on the National Archive, including archived publications.

How did this benefit road users?
The aim of these works was to:

• Allow traffic to flow more smoothly, resulting in fewer accidents
• See a noticeable reduction in road noise in most places immediately after completion and no noticeable change in traffic noise levels over the long term due to the new lower-noise road surface
• Provide improved drainage, resulting in better water quality and safer roads
• Provide new modern "cut-off" lighting, which meets the level needed for a four-lane motorway while minimising the effect on adjacent properties

Following scheme completion, we undertake a Post Opening Project Evaluation (POPE) to assess whether the scheme has achieved its anticipated outcomes. The evaluation is generally carried out at one and five years after scheme opening. The results of the POPE allow us to learn lessons and improve future scheme delivery.

Learn more about the POPE process.

How do I find out more information?
If you have any queries about this project you should contact the Highways Agency Information Line by emailing ha_​info@​highways.​gsi.​gov.​uk or calling 0300 123 5000.