I bike commuted daily for 28 years. I have an AirZound airhorn on each bike. It sounds like a handheld boat horn, but you pump it up with your bike pump.
I know it works even when car windows are rolled up, because you can see the heads turn...and the occasional finger.
On MUPS I can announce my presence way in advance so others aren't surprised.
I also use a bell for close in, polite alerting.