Do you mean the pedestrian underpasses or the underground trains? Because the noise level in an underground train station is significantly less than at street level, it just seems louder because it is intermittent.
The SPL on the London Underground reaches maximum levels of >105dB (previous measurements on the Northern Line hit ~108dB, IIRC). Average noise levels on some parts of the deep underground lines (Victoria, Northern, etc.) can be well in excess of 80dB.
Depends on the bus, but my new “favorite” alert on my Apple Watch is the one that pops up in the tube between San Francisco and Oakland letting me know that it is now 90db and about 10 minutes of exposure to this level of noise can cause hearing loss. I believe it also accounts for the reduction in noise I’m already experiencing from my AirPods Pro, so this is 90db through the noise reduction.
Also it takes about 10 minutes, maybe a little bit less, to get through the Transbay tube. Maybe bring hearing protection if you take it everyday.
You had me questioning whether I was misremembering and I couldn’t corroborate this on the web, but luckily I took a screenshot I think the first time I got this notification back in February because I couldn’t believe it either and Photos has full text search now.
Below is the text from that screenshot:
“Loud Environment
Sound levels hit 95 decibels. Just 10 minutes at this level can cause temporary hearing loss. This measurement takes into account the sound reduction from wearing your AirPods.”