Traffic capacity is the capability of a waterway to deal with the traffic and when the traffic volume exceeds this limit traffic flow stops, as is often experienced on congested roads.

According to the U.S. Government Highway Capacity Manual the capacity of a highway with four or more lanes and free from conditions hindering smooth traffic flow has a maximum value of V/Ymin per lane, where V is the speed of a group of vehicles and Ymin is the average minimum separation of cars. Since Ymin is a function of V and increases rapidly and continuously with speed, there is a maximum value for V/Ymin which is a function of the speed. This is called basic capacity. Several examples of capacity together with the speeds of the transportation facilities are given in Table I.