Most of us have experienced traffic that seems to slow to a crawl or even a standstill for no apparent reason. There are scientists who specialize in studying the dynamics of traffic flow in hopes of finding ways to keep them from happening. A traffic jam occurs when cars go abruptly from highway speeds to a near standstill -- even where there is nothing blocking the road. Traffic becomes prone to jams when there is a very high density of cars on the road, like during a typical commuter "rush hour." When that happens, even a slight fluctuation can interrupt the smooth flow of traffic and cause a bottleneck, or jam. Cars exiting or entering a crowded freeway ramp can just as easily cause a jam as a wreck or road construction.
Even after a bottleneck is removed, it takes some time for traffic jams to clear up. This is because the cars are still locked in a standstill; only the cars in front have anyplace they can move. Even when the car in front of you moves forward, you can't begin moving forward right away. You need to delay a few seconds so you won't travel too close to the car in front of you, which is unsafe. Every car behind you experiences the same delay. So instead of dissipating all at once, the traffic jam gradually "evaporates" starting from the front (where the blockage occurred) toward the end of the jam. The process takes a long time because new cars are constantly piling onto the back end of the jam.