You can't understand it -- how can a five-lane highway become so packed? It feels as if every car in the city has joined you on the highway at the same time. Traffic creeps forward at a snail's pace, when it moves at all. You're forced to waste time, gas and money. What causes this?
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As the first car stops, the following cars must also stop. Even when the first car begins to move again, additional approaching cars have to stop farther down the road, and the congested area travels backward in a wave until traffic is light enough for it to dissipate.
Assuming construction, accidents, and stalled vehicles aren't to blame, it's likely due to more cars entering the highway than leaving it. As more cars enter a crowded road, drivers have to use their brakes to avoid collisions, creating a traffic wave. A traffic wave occurs when cars slow down, and the slowing trend continues backward -- like a domino effect. As long as there are more cars approaching from behind, the traffic congestion travels in a wave.
In general, you can divide up the contributors of traffic into two broad areas: network overload and traffic disturbances.
If there are highways or surface streets that suffer from heavy traffic congestion, no matter what the actual road conditions might be, they fall into the category of network overload. These are the bottlenecks and traffic snarls where demand always outweighs capacity.
As space opens up ahead of your car, you can accelerate and escape the congestion. The person behind you can accelerate a few moments later, and the person behind them a few moments after that. The congestion doesn't immediately clear up -- it continues to shift slowly back down the highway. Congestion can clear if traffic becomes light enough to stop the traffic-wave effect.
Accidents and breakdowns, road construction and repair, and harsh weather conditions are all considered traffic disturbances. You can't always predict where these disturbances will occur, but they still heavily impact traffic flow.
It's easy to imagine construction, an accident, or a cop giving a traffic ticket causing congestion -- drivers slow down either to change lanes or engage in a bit of rubbernecking as they try to see what happened. Road work might shut down one or more lanes, requiring drivers to shift over into open but crowded lanes. Bad weather might cause some drivers to maintain a slower driving speed out of concern for safety. According to the 2007 Urban Mobility Report from the Texas Transportation Institute, traffic incidents account for between 52 and 58 percent of the delays motorists experience [source: TTI].
In the next section, we'll learn about cities and highway traffic.