What Are the Causes of Traffic Congestion
I think that cell phones are a very large cause of traffic congestion. I believe that the biggest cause of congestion is probably construction and accidents. Many people slow down to stare at accidents even after they have some lanes open for cars to get by. Cell phone use is distracting and could be responsible for some congestion, but not most.
Major Causes of Traffic Congestion in the U.S. - Infoplease

What are the main causes of traffic congestion
Cell phones are not one of the biggest causes of traffic congestion. I commuted from Ft. Worth to Dallas and back again five days a week thirty years ago. It was stop and go traffic all day everyday. At that time, I saw one person stuck in the same traffic I was, who was talking on a phone. The rest of us were simply victims of too many cars and not enough lanes and everyone trying to get to the same place at the same time.
The main causes of traffic congestion | eHow UK
One of the main causes of traffic congestion is uncontrolled urban development. Beijing's monocentric urban development has led to the one-directional flow of traffic during peak hours. The urban sprawl, coupled with ever-increasing vehicle ownership and increasing number of commuters, has outpaced the development of the public transportation system. All these have accumulatively contributed to the severe traffic jams we face every day.

Standard workdays are believed to be a main cause of traffic congestion – think rush hour traffic – that is inevitable unless businesses restructure their working hours. Public transportation, in combination with toll roads, may help to decrease the traffic congestion and the funds raised from toll-ways can be rolled into expanding public transportation.