After the wreck is removed, there seems to be no reason for the traffic jam to persist. Yet it does. The reason for this is sensible: if I am stuck behind a car that is stopped, then I have to stop too, and so does the car behind me. All the cars in the jam are in this situation. Even though the wreck is gone, they remain locked at standstill because if they want to move, they ALL have to move at once. They never do, because each driver is waiting for the car ahead to move. If I am in the traffic jam, I'm not going to move forward because I have no room to do so. I'd bump the car ahead of me. We all think like this, so none of us can move.
When the car in front of me leaves, I still cannot accelerate instantly, so I will remain stopped for a moment. I must delay leaving for a moment. If I started up instantly, I'd stay too close to the car ahead of me, and that would not be safe. Each departing car must delay in the same way, and this causes the jam to "evaporate" starting from the forward downstream end. It evaporates in a wave which begins at the forward end of the jam, (near the wreck). The wave eats into the jam from right to left, yet new cars are piling onto the back end of the jam.
Starting at figure 2A, the cars depart from the jam in sequence. In 2B the wave of "evaporation" has moved away from the wreck site, and in 2C and 2D it is far from the wreck. But notice an interesting thing: even though the CARS THEMSELVES are moving from left to right, the "wave of evaporation" moves in the opposite direction. It moves leftwards as it eats into the traffic jam.
There is a second important thing to notice. While some cars are still jammed, more cars are piling up behind them at the trailing end of the jam. Even after the wreck is removed, more cars are still "condensing" onto the back of the jam. The traffic jam is like a solid object whose front end is evaporating and whose back end is growing like a crystal. Cars move left to right, yet look at the the group of stopped cars. The stoppage is creeping slowly upstream, in the opposite direction to the moving cars. The accident is gone, but a "moving wave" of stopped cars remains behind. It's not a traffic jam, it's a shock wave which propagates through the "automotive material". It's a traffic-clot in the blood vessel. It's a travelling wave of traffic-condensation.
These sorts of travelling waves are common during heavy traffic conditions. An accident isn't needed to create them, sometimes they are caused by near-misses, by people cutting each other off, by merging lanes at a construction site, or simply by extra cars entering from an on-ramp. In traffic engineering lingo, they can be caused by "incidents" on the highway. A single "rubbernecker" could cause one by momentarily stopping to look at something interesting. Whenever you slow way down in order to merge across a lane to get to your upcoming exit, YOU could create one.
Sometimes the traffic waves have have no real cause at all. They appear because tiny random motions can trigger large results. They are like sand ripples and sand dunes, and they just build up for no clear reason. They are like ocean waves caused by the steady breeze, or like the waves which move along a flapping flag. They just "emerge" spontaneously from the writheing lines of traffic. In the science of Nonlinear Dynamics this is called an EMERGENT PHENOMENON."
How long will the "traffic wave" last after the accident is cleared? Its lifetime depends upon the amount of traffic, and on the number of cars trapped in the jam, but sometimes these things can persist for many hours. When traffic is slight, the jam might shrink rapidly to nothing. But if traffic remains heavy, then there's no reason for the travelling wave to ever dissipate at all. Also, if the conditions are just right (if the "condensation" happens faster than the "evaporation",) then even a tiny wave could grow large and larger. Sort of like dropping a tiny seed crystal into a supersaturated solution. When traffic is heavy and unstable, slight braking by any driver can cause the traffic to freeze into a gigantic crystal. Like Kurt Vonnegut's end of the world story CAT'S CRADLE it's the "Ice Nine" of the highways.
So, next time you are commuting and you approach a stoppage, don't think of it as a stupid f@#$% traffic jam. Think of it as a pressure wave which has approached your car and engulfed it. Think of it as a simple living thing which is composed of cars rather than molecules. Stay hopeful that the Crystalline Amoeba poops your car out soon. Take an aerial viewpoint, and visualize the wave which is moving backwards as you move forwards.