Measuring Congestion

Traffic Congestion refers to the incremental costs resulting from interference among road users. These impacts are most significant under urban-peak conditions when traffic volumes approach a road’s capacity. The resulting congestion reduces mobility and increases driver stress, vehicle costs and pollution (TTI, 2001). Traffic congestion is considered one of the main urban transportation problems (in this case, “urban” includes suburbs, and even small resort communities during tourist season or other major events), with an estimated cost of approximately $100 billion annually in the U.S., and comparable costs in other countries.

Congestion can be measured in various ways, including roadway Level of Service (LOS), average traffic speed, and average congestion delay compared with free-flowing traffic (“Congestion Costs,” Litman, 2004a). The capacity of a road depends on various design factors such as lane widths and intersection configurations.  Traffic speed and flow on urban streets are determined primarily by intersection capacity, which is affected by traffic volumes on cross streets and left turn signal phases.