M4 J3-12 Managed Motorway

Scheme outline

Upgrade of the existing M4 between junctions 3 (Hayes) and 12 (Theale) to Smart Motorway, with the hard shoulder converted to an extra traffic lane for most of that length.

Unusually for a Smart Motorway scheme, bridge widening and replacement works will be carried out to provide extra space on parts of the motorway that presently carry three lanes with no hard shoulder. The M4 Slough and Maidenhead Bypass sections were subject to "rapid widening" in the 1970s which left the motorway with a discontinuous hard shoulder through bridges.

All Lane Running will be continuous through junctions, with the exception of 3 and 12 at the ends of the scheme, and 4B (M25) and 10 (A329(M)), where the volume of turning traffic will require the fourth lane to be dropped through the junction.

The stated aims of the scheme are to relieve congestion and improve journey times and reliability.

The start of development work was announced on 8 May 2012.

Under construction
Other upgrade
under motorway restrictions
Highway authority
Central Government


What's happening on this scheme? You can add a progress update if you know more. Add a progress update
3 June 2015

The application for the Development Consent Order was approved on 27 April 2015.

5 September 2016

The scheme has been approved by the Planning Inspectorate despite a number of high-profile objections.

21 March 2017

Enabling works for the scheme have started, with average speed cameras being installed along the affected stretch.

More information


With thanks to Barry Salter, TheBowlOfPetunias and Michael Pritchard for information on this page.