Too much traffic for existing roadways to handle is the most common cause of traffic jams. Drivers getting on the highway when traffic is already slow contributes to this problem. Other common causes of jams include traffic accidents, construction, bad weather and overly cautious drivers.
Congestion jams are common when communities grow faster than local governments can upgrade the road infrastructure. Construction projects may eliminate one or more lanes of traffic, condensing the available space for vehicles. Snow and rain cause drivers to slow down. Phantom jams occur from the snowball effect when a driver slows to look at something or to avoid hitting something.