Unfortunately, bike riders use the vehicle for speed.  No one can hear them coming on sidewalks, streets or trails unless the rider vocally warns or has a bell to do so.  Most of the time, at the speed they are traveling, they are already on top of the walker or animal before the victim can react.  Hikers are warned to wear bells, sing, or loudly talk in order to warn potential predators of their presence in trail areas.  Bicyclists, at much greater speed for no good reason, are averse to warning, as they, apparently, believe they can steer their way out of any situation.  Speed is paramount over safety.  Similar to the false belief that a holstered pistol eliminates the need for any other cautionary actions on the part of the human.