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.
The application for the Development Consent Order was approved on 27 April 2015.
The scheme has been approved by the Planning Inspectorate despite a number of high-profile objections.
Enabling works for the scheme have started, with average speed cameras being installed along the affected stretch.
The 2017 works were for bridge strengthening between J12-13 and at J10. Whether they were related to the smart motorway works I don't know.
However, last week a sign was erected just before J12 eastbound proclaiming "Upgrading to Smart Motorway" and with a completion date of Spring 2022.
A construction compound seems to be being built in a field just past J10 eastbound: lots of trenches have been marked out, and a concrete ramp has been built to enable access to/from the motorway.
The start and end dates for various construction points are as follows (and subject to change)
Junction 12 to 11: May 2020 to February 2022
Junction 11 to 10: May 2020 to January 2022
Junction 10 to 8/9: September 2018 to May 2020
Junction 8/9 to 6: May 2019* to December 2021
Junction 6 to 5: December 2019 to March 2022
Junction 5 to 4b: October 2019 to March 2022
Junction 4b to 3: January 2020 to March 2022
* Mobilisation work to 4 structures in junction 8/9 to 7 link will also start in late autumn 2018
- Highways England: M4 Junctions 3-12 Smart Motorway
- BBC News: Hayes to Theale M4 hard shoulder will be widened to become fourth lane
With thanks to Barry Salter, TheBowlOfPetunias and Michael Pritchard for information on this page.