About two weeks ago I became frustrated with people swerving around the highway - because they were texting while driving. As a preventative measure, I have taken it upon myself to drive alongside these offenders and repeatedly honk until they put their phones away. This doesn't usually take very long because most texters know they are doing something dangerous and quickly snap into a hyper-alert state when they hear a car horn.
A quick Google search reveals that I am not the only one who has had this idea.
There is another blog post that is pretty extensive here, which says a lot of things I would have said if it wasn't already online.
I once honked at this guy talking on the phone too - his first reaction was ... nothing, of course, because he was too engrossed in the conversation to notice a honk from a car by his side. A few honks later he kind of looked at me with a "what?.....!" expression. To avoid this awkwardness and save my effort, I decided to focus only on people texting and driving (and excuse people texting at red lights).
So the bottom line is - actively honk at dangerous texters and you will make the world's roads and sidewalks safer.