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 can only be achieved under specific conditions of traffic speed and density:

• Critical Density. The volume generally increases with the density of the traffic. However, at some point the density is too high and congestion decreases the volume.
• Critical Speed. The volume generally increases with the speed of the traffic. There is however a speed at which the volume is maximal.