When does traffic congestion happen?

Traffic congestion is a critical problem which happens on roads which make traffic busy because roads are full of cars and buses. Traffic congestion challenges traffic flow in urban area and is prevented smooth traffic. A growing urban area creates complex problems in daily life with traffic. It is necessary to build technology system for transportation management which is used for control of the traffic phenomenon. Traffic control systems have direct influence on traffic problems which is help to improve traffic flow and reduce traffic congestion. Approximately, traffic congestion occurs in the rush hours in the morning or evening when people are travelling to work. In addition, transportation systems are one of the main reasons causing traffic congestion in some countries. Traffic roads have a direct effect on traffic jams which relate to small ways use or damaged ways.
1. When does traffic congestion happen?
2. What area has complex problems with traffic?
3. Do we need traffic control systems?
4. What directly effect on traffic jams?