A wave made of cars
Why Is There Nothing at the Front of a Traffic Jam?
You crawl for twenty minutes, reach the front and find empty road. Nothing crashed. The cause was never at the front — it was behind you all along.
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Step 1 of 5
One driver brakes, and the one behind brakes harder.
Nobody reacts instantly. By the time you notice the car ahead slowing you have already lost some of your gap, so you brake a little harder than they did to win it back.
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Step 2 of 5
Heavy traffic has no spare room to absorb it.
On an empty road the gap ahead of you is enormous and a small slowing simply eats a little of it. In heavy traffic there is no spare gap, so every slowing has to be passed on.
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Step 3 of 5
So the slowing travels backwards down the line.
The same small event, arriving larger each time. Each driver brakes slightly harder than the one in front, and forty cars back somebody comes to a complete stop for something that was never more than a lifted foot.
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Step 4 of 5
The jam moves the opposite way to the cars.
Which is exactly why the front of it is empty. The cars creep forwards. The stopped patch itself slides backwards along the road at about walking pace, swallowing new cars at its tail and letting them go at its head.
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Step 5 of 5
It survives because nobody sets off at the same moment.
A loop that feeds itself for hours. Getting going takes a moment of attention, and each driver spends that moment only after the one ahead has spent theirs. The delay is handed back down the line as fast as the road hands cars in.
Traffic engineers call this a phantom jam. It is why holding a steady gap in a queue clears one faster than closing every gap the moment it opens.
The short version
A jam is a wave rather than a place: one small braking grows as it passes backwards down a line of cars with no gaps to spare, so the front of it is open road.
Try it yourself
Next time you are in a crawling queue, watch the brake lights ahead of you. You will see them switch on in a line running towards you.