That area of the M4 has been a nightmare for years. Too much traffic compressed into an inadequate road. All it takes is a minor issue and it is mayhem.
Fire in the tunnel caused a massive problem a couple of years ago and the tunnels are now having upgrades resulting in overnight closures.
The plan/route for a new road has been ongoing for years and no final route set yet.....lots of objections etc.. meanwhile the chaos continues.
The "smart" road management system apparently monitors traffic volume and lowers speed limits as it builds up to improve safety (?). OK more traffic means possibly more incidents but surely squeezing vehicles up doesn't help. The impatient bully-boy drivers are a menace there, jumping lanes, ignoring lane closure warnings, tailgating. It is not a pleasant area to drive through.