That you can tell the exact moment when someone in a car decides to pick up a cell phone and start yakking away.
It's the exact same moment that a reasonably sane driver turns into a fuck-witted asshat of the highest order. You can tell when this occurs because they suddenly can't stay in their lane, or maintain the same speed. They also ignore everything else around them, making them a danger to every other car on the road.
Yes, this happened to me yesterday. Why do you ask?