Old thread I know, but I think its an interesting and useful topic. Strait from my anatomy class: human hearing has both an absolute threshold and a relative threshold. The lower absolute threshold is very low, meaning we can hear very soft sounds (about 10 db). The harder part is the relative threshold- distinguishing one sound level from another. Generally, each source of noise drowns each other out anyway and you only hear the loudest source (90 db dump truck) at one time. My commute is at rush hour on a bike lane beside a road used by cars, buses and lorries so I think even the softest times will be over 50dB (Light automotive traffic at 30 metres. I wouldn't use earplugs on quiet roads and pathways but where the noises constantly exceed 80 or so dB, I think it will serve as sunglasses for the ears. Likewise, it would be dangerous to wear sunglasses when its dark but when its bright, sunglasses reduce the brightness of everything- into a comfortable range.

All this is just theory though. I'll try it out when I ride tomorrow.