So that person in the car/truck/bus just cut you off and forced you onto the shoulder. Should you give them the middle finger wave to let them know what a jerk they are? Not IMHO. You should get their attention and alert everybody in the block that there is a dangerous driver. Blast them with an airhorn.
So where do I get and mount an airhorn? Well Amazon sells it here:
And this is how I have mine mounted on my bicycle.
And this is how I use it,
I just roll my forearm onto the switch while maintaining a grip on the brakes. You can just reach over and tap that horn but if the car makes a sudden move you might also want to apply your brakes.
I have found that a sudden horn surprises most motorists. First they have no idea where its coming from. Second they tend to look a bit sheepish after getting "caught" cutting you off.
The second place I find this horn useful is coming up to an intersection where cars are thinking about taking a free right on the red light. A light tap on the horn tends to help them see me and not continue rolling into the intersection. Not a full blast, a couple of light taps.
The third place is passing a bus which has stopped to pick up passengers. I shift to the left lane and as I come around I give them 2 light blasts. This is the same signal that bus drivers use with each other to alert each other that they are coming around.
The last place I use this horn is with pedestrians. It's really too loud and scares the bezejous out of them. So either hit the horn lightly well far away or ride up slower and use your voice.