You know when you're stuck in traffic, and cars are moving for a couple of seconds, then they stop again? If you look at that view from a height, you will notice that it resembles shock waves, and hence the term. We all experience them all over Cairo, at any time of the day, and in any street.
What have we done to improve our traffic situation? We bring "foreign experts". Or, the other option is that someone from the Cairo/Giza traffic comes up with an idea. However, scientists and mathematicians are modeling shock waves in very interesting experiments. One of them was that Japanese scientists asked some volunteers to drive in a circular track, at a certain speed. It was noticed that humans cannot maintain the exact same speed, causing some cars to slow down a bit, cars behind them used their brakes, and so on, and so forth, until it caused a traffic shock wave, causing some vehicles to come to a complete stop. No accident up ahead, no check point, not a car broke on the side of the road, nothing except pure shock waves.
My contribution to this theory, is that shock waves do not happen all the time. I mean, if you're driving @ 2:00 am on a highway, you will rarely experience traffic shock waves. Hence, there has to be another factor, which is the density of cars. Not density of the cars' materials, but the number of cars in a given area (width of the road, and any arbitrary length). For a certain road, there is a certain threshold density, that if reached, shock waves begin to formulate. Since the length taken for our sample is arbitrary, then the density is directly proportional to the width of the road.
Here's some food for thought, think about that next time you decide to squeeze your car in a fifth lane in a 4-lane road.