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Why Do Traffic Jams Appear Out Of Nowhere?

Author: Jason Snyman
Date: 2019-07-28
Does it sometimes seem as if traffic jams just appear out of nowhere? Believe it or not, but there's actually a science behind it, and we've got nobody to blame but ourselves.

For the last couple of years, various traffic indices have named Cape Town as the most congested city in South Africa. Commuting in and out? Forget about it. Bring a magazine. Taking the train? If you want to be nine hours late for work. Try a Microlight instead. With 260 000 vehicles entering the Mother City daily, the city has acknowledged it does not have the capacity to sustain the surge in private vehicles on its roads.  

Still, it has long been debated which South African city truly has the tougher traffic conditions. Each major city has its own major flaws and likewise, the residents of those cities behave and think in different ways. 

If you’ve ever lived in Johannesburg, you’ll know that it only takes one malfunctioning robot on Beyers Naude or William Nicol to turn a one-hour commute into a three-hour horror show. If you’ve ever lived in Durban, you’ll know the true meaning of sluggishness. Durbanites crawl down the highway as if it's a parking lot. Pretoria residents know the sinking feeling of taking one wrong turn all too well. With all those one-ways, a single error will force you all the way around four city blocks, and maybe, even, into an oncoming bus lane. If you’ve ever lived in Cape Town, well, then you know all about the benefits of Prozac. 

Let's go a little deeper into the reasons behind the gridlock in these cities, and how we, as motorists, can do our part to fix the problems. 

The Braess Paradox and Nash Equilibrium

Contrary to common sense, it has been suggested that the closure of roads can actually increase the flow of traffic. This is known as the Braess Paradox. 

Rush hour is a non-cooperative game, a collective decision brought to life by inherent selfishness. Every driver is trying to minimize the time they spend on the road. The problem is, every other driver is doing the exact same thing, and so the travel time remains the same for all drivers on every route. This equilibrium which is achieved is known as the Nash Equilibrium. 

The Nash Equilibrium suggests that we settle on a route once we're convinced that switching routes won't result in a faster drive. This diagram demonstrates how the mathematics behind it works: 

So, in essence, new roads do not fix traffic, but often make it worse instead. An extension of a road network by any additional road, as shown in the above diagram, could cause a redistribution of the flow in such a way that the travel time increases. As demonstrated above, there will always be an optimum solution (dividing evenly between routes C and D) to decrease the total amount of travel time for all drivers. 

But, being as we are, would we risk the chance of some other person taking the A-C-D-B route and arriving at their destination before we do? Why can’t that person be you? That kind of thinking is often why we spend unnecessary, extra time stuck in traffic.  

The Zipper Merge and Fluctuation

Unless you’re moving at highway speeds, the safest, most efficient way to merge is to use the Zipper Merge. This is done by waiting until the last possible moment, and then taking turns merging into the lane with other drivers in a ‘zipper’ like fashion.

Many drivers will see the ‘lane merge’ sign in a construction zone and panic. They slow down too quickly, perform unexpected, dangerous lane switches in order to get into the appropriate lane and ultimately cause serious accidents. Perhaps you've even been guilty of this yourself, or found yourself forced into making these decisions due to other inconsiderate drivers. 

Many nice motorists feel bad about merging at the last moment. They might feel that the cars in the continuing lane have the right of way, because they were there first. Zipper merging, however, benefits individual drivers as well as the public at large. This system gives the merge order and keeps us safer. It reduces differences in speeds between the two lanes, as well as easing congestion. 

Things move a lot smoother when there’s a sense of fairness about it, that everybody is moving at the same rate. The biggest culprit in any traffic jam is, after all, sudden fluctuations in speed. At a constant rate, traffic flows freely. When a single driver slams on the brakes in order to merge, or slows to rubberneck alongside an accident site, this causes a chain reaction. 

Slowing down, speeding up, slowing down, speeding up – the flow is lost.  

Vehicles enter the back of the jam at a faster rate than the vehicles in front can accelerate, creating gridlock – all because of one person and their inability to brake properly, or curb their curiosity.

The Treadmill Effect

Let's say that you’ve been running on a treadmill at the gym for some time. Then you stop and get off, and the room appears to loom toward you. Some of us may have experienced this feeling before. This is because the neurons in our brain, which track forward and backward motion, grow tired. 

The same thing happens to us on a highway. 

We drive at a high speed, zone out as we stare at the road unfolding ahead, and then suddenly have to slow down as we approach a traffic jam. We have trouble adapting to this - we think we’ve slowed down more than we have. The brake lights of the car ahead grow far too close, far too quickly. Before you know it, you’ve created a backward-travelling wave down the highway, affecting people long after the incident occurred, as they're forced to slow more and more until eventually, everything grinds to a halt.

When driving, it's important to keep our eyes moving, and keep ourselves focused on the task at hand. 

Traffic Congestion

Heavy congestion may sometimes seem as if it materializes out of nowhere. The simplest explanation for this is that there are just too many cars, headed to the same areas at the same times, and not enough road. 

While this is true to an extent, the larger truth is much more disappointing and illogical. 

Traffic is a strange phenomenon, compounded by our own human instincts. While some people brake and weave for no apparent reason, we also have a genuine, human inability to maintain a steady, constant speed. Traffic jams, sooner or later, will materialize. 

This is all without taking into account that a number of outside elements could also influence the flow. Construction, malfunctioning traffic lights, bad weather, potholes, roadblocks, breakdowns, etc – these are all contributing factors which are completely out of our control. If the people in front have given in to their curiosity and hit the brakes in order to idiot-gaze at a roadside accident, a fire, flashing lights or Bigfoot, or whatever – we have no control over this. We, too, are forced to brake in order to avoid rear-ending the car in front of us. It all comes down to one person giving in to their basic programming. 

In the end, traffic gets as bad as it does because we, as people, cannot understand the larger traffic system. Until self-driving cars enter the mainstream, which is very unlikely to occur in South Africa any time soon, traffic will always be a problem.

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