I joined this forum, because I am both a biker and a pedestrian (long-distance walking etc.). I would like to mention something that seems obvious but that many bikers overlook: the timing of your warning (bell or shout) to pedestrians in the path ahead of you. I know some elderly people (slow reaction times) who have been knocked down by bicyclists, and I have also experienced some near misses and brushes with speeding bikers as well.
If you are biking at 20 mph (not a very fast pace), you are covering about 30 feet per second. That means if you shout or ring your warning from 30 feet behind the pedestrian in front of you, s/he has one second to process what you shouted (or dinged) and react to it. If you do it 15 feet (3 bike lengths, shall we say?) from behind the pedestrian, s/he will have one half of a second to respond.
Neither of these is really enough time, but too many bikers wait until they are actually passing before saying, "On your left" or ringing a bell. This is more likely to startle the pedestrians than give them warning, causing them to jump into your path or move erratically in front of you.
Not the intended outcome.
I suggest bikers start ringing their bells or shouting their warnings from AT LEAST 50 feet away, especially if they are traveling faster than 20mph. It sounds like a PITA to do that for every single pedestrian, but if you cannot be bothered, don't complain about pedestrians in "your way."