You're driving to work and look over to the other lane to spot a colleague also on his way to the office. You could honk your horn or flash your lights, but you have a cell phone. So you pick it up and dial the guy. And, while you're watching, he hears his phone ring, reaches for it, looks at the caller ID, and then ignores your call. Perhaps he's concerned about the safety issue of talking on the phone and driving at the same time? Either way, now's the time to start honking and flashing your lights. You got him.