Hi,
We have a craze amongst the kids in England for incredibly powerfull, bass heavy car sound systems playing at max. volume. The cars shake and bibrate even when at a standstill.
I'm not even in that league, probably I'm getting boring with old age, but do have a 12 speaker HK CD set-up.
I fail to see the diiference when I am playing music at high volume, in a car that is well sealed, and canal 'phones.
In neither case can you hear outside sounds. If you check your mirrors frequently, then you can easily spot an emergency/police vehicle coming up behind you. I do this anyway since I am always breaking the speed limit when I consider it safe to do so.
A lot of motorcyclists need to seal their ears on long journeys, using ear plugs, because of wind noise. Canal 'phones are also frequently used.
However, I agree that I would not use them on a bicycle or when walking in town.
regards
John