Critical Density and Critical Speed
The capacity of a road segment is commonly defined by the number of vehicles it can handle over an 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 some other figures place it at around 70 km/hr.