Carry some water (or any other liquid) with you.
As you pass by his side, throw the water inside his window to wet his dumbphone.
I hate these morons who text in traffic.
Nice thought, most likely not going to work.
On the unlikely chance you'd even hit his phone with the water; most phones come with a water-resistant coating these days.
Even my 'cheap' Moto G, look on youtube, there's a video of a girl who left it underwater for 30 minutes and it was just fine.
(I've used it in a downpour myself, no worries)
Back to topic...
This doesn't involve traffic, but it's a good 'moronism' I think.
When I was working at Wal*Mart, my department (Fabrics and Crafts) has its own register. We cut, we collect.
One of my co-workers NEVER would fill the bag rack, or get change for the register.
One night I went in and both were basically empty. Not one plastic bag and only one or two $1 bills in the drawer.
So I finally had it and said, "Why is it that no one
can ever get change or get bags?"
She actually said, "Yeah, I know; right?"
I looked at her and said, "I WAS TALKING ABOUT YOU! You were the only one here!"
"Oh, sorry, give me $100 I'll go up front and get some change and bags..."
After that, there were always bags and change, but she didn't talk to me much......not that I cared, I at least had change to give my customers and bags to put their merchandise in.
I think I scared her. I was like a chihuahua barking at a pit-bull, but it worked. LOL