A typical daily urban commuting profile involves two peak hours, morning and afternoon, which are recurring, particularly on weekdays and less so on weekends. Under normal circumstances, the transport system has a fixed capacity that remains constant throughout the day. This capacity is an outcome of road characteristics such as the number of lanes and operational constraints such as the mix of vehicles and speed limits.
Congestion occurs when the amount of traffic exceeds the design capacity, which is commonly the place during the morning and the afternoon peak hours. An issue relates to the level of capacity that should be provided considering the daily distribution of traffic. A capacity that is too high would be expensive to provide, although it would offer the benefits of limited congestion. Limited capacity would be linked with enduring congestion and negative economic externalities.