How is a traffic jam like a lightning strike?

When we look at things move, we intuitively gauge their direction. But what if our intuition is wrong?

More than just light connects traffic to lightning.   David Alonso/Flickr  (CC BY-NC-ND 2.0)

More than just light connects traffic to lightning. David Alonso/Flickr (CC BY-NC-ND 2.0)


This isn’t an undiscovered Edgar Allen Poe piece, although you’d be forgiven for saying they’re as similar as a raven is to a writing desk.

But both traffic jams and lightning strikes hold within them a secret — both flow in ‘reverse.’ To understand what I mean by that, let’s take a close look at specific type of traffic jam: the phantom jam.

A phantom jam is a traffic jam without an obvious cause, and they happen because of individual drivers’ reactions to the signals in front of them. All because a driver — whilst rounding a corner or perhaps allowing someone to merge — slows down slightly. The driver behind them sees the brake light and taps their own brakes in response. And so on down the line of traffic. Eventually, someone comes to a complete stop.

They quickly start moving again, but it’s already too late. The car behind the stopped car has itself stopped, and the one behind them, and the one behind them. This stop travels backwards down the line of traffic, sometimes going for kilometres before traffic thins out. 


The phantom jam, here explained by the BBC's Andrew Marr, is furiously studied by city planners trying to reduce commute times and accidents on the road.


This phantom jam is an example of an 'emergent phenomenon,' something that can only be observed at a systemic level, and could not be predicted from the motions of an individual in the system — in this case, a single car. It’s a jam without a cause, created (but unseen) by the drivers within it. When we look at the cars, we see them travelling forward along the road, and it’s natural to assume that a traffic jam would also move that way. Looking at the system as a whole, we see that because of the nature of the jam, it flows back — not forward.

Traffic moves slowly. It’s something we can see — something we experience personally — and so it’s easy to make observations like this. But how can we do the same for lightning, when it moves at over 351 million kilometres per hour? To think about how lightning moves, we need to first understand electric charge.

Electricity, as physics understands it, is the flow of electrons from areas of negative charge, where there are many electrons, to areas of positive charge, where there are few. In a cloud, the friction between dust particles generates an excess of electrons, creating a strong negative charge. Eventually, this charge becomes too strong to be contained in the cloud, and small streams of charge — known as ‘leaders’ — are forced out of the cloud.

These leaders make their way to the nearest positively charged area, which is usually the surface of the earth (but may be a nearby cloud). As this charge travels towards the ground, it ionises the air through which it travels, freeing electrons and creating a pathway for them to travel. As it nears the ground, a positively charged leader rises up to meet it. When the two connect...

The observed direction of a lightning strike is against the flow, not with it.   Koba-chan  /Wikimedia Commons  (CC BY-SA 2.5)

The observed direction of a lightning strike is against the flow, not with it. Koba-chan/Wikimedia Commons (CC BY-SA 2.5)

KRRRABOOM! The excess electrons in the cloud rapidly discharge along the pathway into the earth. So quickly, in fact, that they superheat the air around them, creating the familiar explosion of light and sound we call a lightning strike. With all that charge moving downwards, you would expect the observed strike to travel from cloud to ground... but like in traffic, things aren’t always as they seem.

What we perceive as a lighting strike is actually just a change in motion. Think of a field of grass, swaying in the wind. Observed from afar, waves seem to travel across the field, but when we look closely, the grass isn’t travelling at all. As individual stalks of grass change direction, we see a 'wave' of grass — composed of a group of grass stalks making similar changes — move across the field. 

The first electron to travel the newly-opened path isn’t the one at the top. To move, the electron must have somewhere to move, and it’s crowded up there in the clouds. Down at the bottom, however, there’s just been an opening. The electrons at the tip of the negative leader are free to move down the positive leader, and this leaves rooms for the electrons above them. These move down, and then the next, and so on up the path, not down.

This motion, like our phantom jam, moves backwards to the flow of electrons. While the individual electrons flow down, the section where the electrons are moving, flows up. So instead of jumping from the cloud to the ground, the visible discharge — known as the return stroke — jumps from the ground to the cloud. This happens too fast to be seen with the naked eye, and we need an extreme slow-motion camera to observe the direction of the strike.


Again, if we were to just assume the direction of the strike from our intuition — or from what can be predicted from the movement of the individual electrons — we would find that our predictions would be backwards.

But why do these phenomena operate seemingly in reverse?

The answer lies in the nature of the ‘motion.’ What we are seeing is not an individual object moving, but rather a change in the motion of a large number of individuals. It’s this change that produces our jam and our strike, but nothing is actually moving in that direction: it only looks that way. Our brains, which are used to observing the motions of objects, interpret this wave as a moving thing, with form and direction, despite its true nature.

Intuition aside, by giving a direction to these changes, we can predict how they are going to affect the systems that create them, and work to prevent these effects. 

Traffic engineers, such as William Beaty, have used their observations to develop countermeasures by changing the habits of drivers. Variable speed limits, which uniformly slow traffic, reduce the chance of random braking developing into a phantom jam, and speed up traffic overall.

Likewise, meteorologists and electrical engineers have used data on return strokes to develop more effective lightning protection systems. Instead of a single high point, which can create positive leaders, a system of multiple points and a safe discharge pathway to the ground can prevent damage to the structure (and its occupants!).

Our intuition is a powerful tool for predicting the behaviour of systems, but we should always be aware: sometimes, the true answer lies in the opposite direction.

Edited by Tessa Evans and Bryonie Scott