He's specifically harassing people who are literally stopped:
“Then he sees it: a driver idling in the late afternoon gridlock while scrolling his phone. Perfect.”
Stopping someone committing a crime is not harassment.
That caught my eye too. Is it illegal in the UK to look at your phone while stopped at a light? If so that's a very silly law, it hurts nobody to look down while the car isn't in motion.
What do you suggest, he tries to get alongside moving vehicles with drivers that aren't paying attention?
Using your phone at all while driving in the UK is illegal, stopped or otherwise.