There is a post in Pistonheads where noise detection cameras are being tested. Similar to speed cameras they would snap an offending vehicle. I don’t know what the trigger level is. I would guess that the limit would have to be a broad compromise due to noise outputs within Construction and Use specs for cars and commercials. The calibration would be interesting to see as well. Average over a predetermined time? Instantaneous? Will the MOT be changed to test each car versus it’s stated spec or versus a limit, as per emissions? The IVA test for kit cars has such a test. It is done outdoors in an open area with nouse meters at prescribed positions.
The M4 section that is raised as it goes over Port Talbot has always been a 50mph zone from opening to reduce noise levels for the houses below it. I broke down on it once! Scary due to nowhere to move to. The hard shoulder had a 4’ wall with a big drop beyond. It was noisy and I had to shout down the phone.
