The shortest measurable unit of time is the interval between the light turning green and the car behind you beeping.
On September 11, 2009, I was stopped at a red light, listening to the radio. The station was featuring audio from September 11, 2001, and they started playing a 911 call from a woman trapped inside an elevator at the World Trade Center.
It's so hot in here, she said. I can't breathe. I'm burning up. I'm dying, I'm dying. The caller's voice got softer and softer as the dispatcher told her to hold on, everything was going to be alright.
I hung my head over the steering wheel and started to cry as I listened to this woman die. I don't know how long I sat there, but when I looked up the light was turning from yellow to red. I'd sat there for at least one light cycle.
I checked my mirror, and there was a long line of cars behind me. No one beeped.