I'm really surprised someone hasn't already jumped in here to proclaim how unsafe hearing protection can be while riding - 'you won't hear or be able to audibly distinguish between safe traffic and drivers who deliberately or by accident will run you over as they pass in their cars.'
Beh, while not being able to hear cars coming behind you might be an annoyance, "distinguishing between safe traffic" is just fear mongering.
How exactly do you hear a car that is going to run you over and one that isn't? Are you going to jump out of the way of every car that seems like it's going to pass close?