1Two things about every driver
A tiny wait, and a gap
To see how a jam is born, you only need two everyday ideas. Watch each one move:
The tiny wait
You don't brake at the exact same instant. When the car ahead slows, your eyes see it, then a beat later your foot moves. That little wait matters.
The gap
The space you leave to the car ahead. A big gap gives you room to slow down gently. A tiny gap means you have to brake hard and fast.
2How a slowdown gets passed back
Pass it on harder, or pass it on softer
When one car slows, every car behind has to slow too. The only question is whether each one slows a little more or a little less than the car ahead. Here are the two cases:
Pass it on harder
Cars are close, so each driver brakes a touch harder to feel safe. The slowdown grows bigger as it travels back.
Pass it on softer
Cars have room, so each driver can ease off a touch softer. The slowdown shrinks as it travels back, then fades.
3Your turn — drive the ring road
A circle of cars, going round and round
Here's a road shaped like a circle so the cars never reach an end — they just keep flowing. Drag the gap to see the road get packed or roomy, and watch how smoothly traffic moves.
Cars getting past per lap: smooth
4Now make the jam
One car taps the brakes… once 🛑
The circle is flowing smoothly. You're going to poke one car — make it tap its brakes a single time, then let go. Nothing is in the road. What do all the cars behind it do a little while later?
Guess before you poke it
You'll tap ONE car's brakes once on a packed circle, then let go. A minute later, what happened to the line of cars behind it?
Road status: flowing