Why Do Traffic Jams Sometimes Suddenly Disappear?

Traffic jams are an everyday occurrence and with the volume of traffic ever increasing, so does the risk of a jam. Ever been in a jam that suddenly dissipates with no apparent cause for its occurrence or resolution? For those who make their living from courier work, the way a traffic jam often resolves itself must be a puzzle. Here’s a theory of how this can sometimes happen.

It’s An Accident

Suddenly, and annoyingly for courier workers on a time schedule, the traffic slows significantly. Then, as soon as the traffic slowed, it speeds up again and you see no reason why it disappeared. Behind you though the traffic is still moving at a snail’s pace. Why? Why don’t they all put their foot down and move at the same faster speed? And why did they slow down in the first place? As a courier driver, you will have no doubt experienced this many times, but have you ever questioned it?

Think about it as if you were hovering above. An accident has occurred and has been moved. It takes a long time for the stopped cars to get moving again, meanwhile other cars are joining the jam at the back of the line and the congestion creeps backwards. Why though, once the incident has been removed does the jam persist?

If you, in your vehicle, are stuck behind a car that is stationary, you have to be stationary too, and so do all of the cars behind you. Although the solution is for everyone to move at once, no one can move because everyone needs the car in front to move first. The driver of the car in front thinks the same and so on.

Eventually the car ahead moves because he can, but there is a pause before you gather yourself to follow. And to keep your distance from the car in front there is further delay as you let it accelerate away. Every driver before you responded in this way, as must everyone behind you. Eventually the jam appears to dissipate and the traffic gathers speed starting with the drivers at the front. As more cars join the traffic at the back, the dense block of congestion moves backwards. The wave of the cars able to move also moves backwards as the freed cars move forward. This ripple is the consequence of an incident and it continues causing issues long after the incident has gone.

It’s Not An Accident

But what if there is no accident? Why does traffic build up? Bad driving, Variable Speed Limits, a truck overtaking and the joining of lanes cause traffic to slow down. Sometimes these traffic waves are caused due a small disturbance in the traffic flow that escalates. This is known as an Emergent Phenomenon in Nonlinear Dynamics.

How Long Will The Wave Last?

This really depends on several factors including the volume of traffic. The traffic wave can last hours and be infuriating for all drivers, not just courier workers. If traffic remains heavy and the vehicles continually join the back of the jam, the evaporation wave keeps moving backwards without dissipating. And, if traffic is condensing at the back of the jam quicker than it evaporates at the front, the wave could become even bigger.

Next time you are in a jam, think of it as a wave of traffic, rather than a stationary queue. The cars are like water molecules that become enveloped in a ripple as a consequence of a disturbance in the water. Nice theory for courier workers to consider isn’t it?

Norman Dulwich is a correspondent for Courier Exchange, the world's largest neutral trading hub for same day courier work in the express freight exchange industry. Over 4,000 transport exchange businesses are networked together through their website, trading jobs and capacity in a safe 'wholesale' environment.

Writer and Online Marketing Manager in London.