Since pedestrians have right of way over cyclists, I consider it rude to ping them, no matter how slowly they are walking. If I am on the sidewalk, or MUP, both situations I try to avoid like the plague, I suffer peds gracefully. I troll along behind them until they notice me and move over, or not, as they will. I am assured of getting a chance to pass them soon, with, or without, their active cooperation. Bells are for signalling other cyclists that already see you, especially children.