It's easy to spot when someone is on their phone while at a light. Look in their side and rear view mirrors and watch their head as they glance up and down. If someone's obviously on their phone and not moving when the light turns, I'm honking.
Same goes for someone engrossed in conversation w/a passenger and very obviously not looking @ the light/road. Or, folks who decide that the red light is suddenly time to re-arrange their interior and are leaned over so they CAN'T see the light/road.
It's not necessarily that MY particular reason for travel is more important than whatever you're doing that's causing you to not pay attention to the road/lights. However, the reason for travel of the 2, 3, 5, 10+ cars behind you collectively ARE more important than your text, tweet, or finding your favorite song on iTunes before you get going.