Update on M4 upgrade, junctions 4 to 12

Published: 06 December 2022

The national speed limit has now been restored between junctions 4 and 8/9 of the M4, and this marks the lifting of the final restriction on the upgraded motorway between junctions 4 and 12.

A section between junctions 3 and 4 in West London will remain at 60mph to help manage air quality in the area and to manage traffic going into London, otherwise the upgrade and calibration of the technology is complete.

The system of inter-related features and technology used on upgraded motorways includes:


  • variable speed limits to help keep traffic moving
  • orange-coloured emergency areas set back from the road are signed and have telephone links direct to control rooms 
  • detection systems to monitor traffic for changes in flows 
  • Stopped Vehicle Detection (SVD) technology which can identify a stationary vehicle, typically within 20 seconds
  • CCTV cameras to monitor and manage congestion and incidents, where notified; the system has the ability to see 100% of the carriageway
  • signs and signals to alert drivers to hazards ahead
  • red X signals to close lanes to other traffic when a stopped vehicle is identified
  • enforcement cameras to deter those who break speed limits and ignore Red X signals 


There will be some overnight lane and slip road closures for the next few weeks while equipment associated with temporary traffic management is removed.

More about the main features of smart motorways as well as the guidance and advice on safe driving and what to do in an emergency may be found on the National Highways web site.