It is a real problem with motorcycle riders; I imagine it's a similar problem with high mileage cyclists, but less discussed, since people think hearing loss is a natural part of aging. But it isn't! At least, not to the degree we're talking about here.
At 20mph, I don't know offhead. But I do remember a thread in the commuting forum where someone taped a lapel mic to his chest while riding at around 21mph or so, and measured 105 decibels. That's definitely enough to produce hearing damage over time. The damage threshold is 85 decibels.
As a recording engineer I can tell you that you do not measure sound pressure with a microphone. You do it with a Sound Pressure Level (SPL) meter. So that figure about 105 dB is absurd. A jet engine produces 110 dB, levels above 85 dB can be damaging for sustained periods of time. I also believe that even at the speeds above 40 mph you are probably receiving about 40-60 db max to your ear, which is the same as a conversation.