What are some alternatives to fruitlessly ringing my bell at a motorist? I doubt that motor-vehicle drivers can hear the "ding" sound that a bicycle bell makes. What are some better alternatives?

I'd go for the air-horn, for example the AirZound.

It is my opinion that screaming and yelling (the primal scream) can cause a lot of unnecessary social distress, and is not a good alterntive for traffic communication and signalling under normal conditions. It ends up being more effective when you're in "panic" as said, which is barely a day-by-day acceptable situation to be.

Air horns, on the other hand, save your throat, are MUCH louder, and have a more emotionaly neutral sound, adequate for signaling in traffic, specially heavy urban traffic.

One single thing is paramount, based in my experience with bell-rings: the activation lever must be easily, instantly accessible by your thumb when the hand is in braking position. If you have to move or reposition your hand in order to activate the horn, it will be underused and practically uneffective.

Hope this helps!