The Brake-Light Ripple

Scenario. Heavy, fast traffic on Mombasa Road. Far ahead, you see brake lights come on, then more, spreading back towards you one car at a time. The car directly in front of you hasn't braked yet.

The clues. Brake lights several vehicles ahead, not just the one in front. You only see them if you are looking far enough ahead and sitting where you can see past the vehicle in front (Look Far Ahead, Position to See and Be Seen).

What could happen. A brake-light ripple is a stop travelling backwards through the traffic. Each driver reacts to the one directly in front, a moment late, and brakes a little harder than the driver before. By the time it reaches the back of the queue, the braking is at its hardest and the gaps are at their smallest. The Swedish Theory Book lists multi-vehicle collisions caused by insufficient distance as a signature risk of fast roads. The ripple is how they happen.

What to do.

1. Lift off the accelerator the moment you see the first brake lights ahead, not when the car in front brakes. You are reacting to the start of the ripple, not the end of it.
2. Check your mirror. If someone is close behind, the handbook's advice applies: slow earlier and more gently, so they have time to react to you.
3. Brake smoothly and early, using the gap you kept. The 4-second gap from Following Distance in Practice is what makes this possible.
4. Stop with room in front, as An Escape Route described, so you have somewhere to go if the driver behind doesn't stop.

The driver who sees the ripple early barely needs the brakes. The one who waits for the car in front is part of the pile-up.