The capacity of a road segment is commonly defined by the number of vehicles it can handle over a unit of time, such as 1,000 vehicles per hour per lane. This volume can only be achieved under specific (optimal) conditions of traffic speed and density:
- Critical Density (or optimal). The volume generally increases with the density of the traffic, since there are more vehicles per unit of surface. However, at some point the density reaches a critical level and congestion starts to negatively impact volume. This is around 1,800 to 2,400 vehicles per lane per hour.
- Critical Speed (or optimal). The volume generally increases with the speed of the traffic. There is however a speed at which the volume is optimal. On a regular highway, the optimal speed is around 90 km/hr, while other figures place it at around 70 km/hr.