Both. Definately. In commuting urban for a number of years a whistle always commanded respect at intersections and w/inattentive drivers. A loud bell works best on MUPs as it's a sound most associated w/bicycles. And not the single chime type. Get one that sounds like the kind one used as a child, but louder. Can't remember who makes them. A German or Dutch company, I believe. Mine had a fire engine motiff...can't remember the name of the manufacturer, but it was loud. Had a wrist band whistle used by basketball refs. Very quick user friendly. And didn't have to ride w/anything dangling around.