People use their ears to navigate traffic (as non-car-users) much more than they realize. There's a reason kids need to be drilled in "look both ways before crossing the street" - you can hear that there's no car coming, what's the problem? There's a reason electric cars need to make that strange noise so you can, in fact, hear them coming. Absolutely a headphone user, with not only ANC to reduce external noises but loud music to mask them, is missing a primary sense for navigating traffic. Absolutely these things increase accidents from minor (someone walking into the path of a cyclist on a multi-use path, oblivious to bells and callouts) to major.
But can that bell penetrate loud music? How many people really walk around with ANC headphones just as a "cone of silence" device?