Other than rush hour driving, the two main sources of traffic congestion are

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Bottlenecks, such as narrow roads or merging lanes, and traffic incidents, such as crashes or breakdowns, are frequent causes of traffic congestion. Work zones and bad weather can also contribute, but they are not as consistent or widespread as bottlenecks and incidents. Special events and inadequate traffic lanes can cause congestion, but they are more localized and predictable than bottlenecks and incidents. Traffic accidents and distracted drivers are both factors that can lead to incidents, but they are not as broad of a cause of congestion as bottlenecks and incidents.

Other Than Rush Hour Driving, the Two Main Sources of Traffic