on the road was at a maximum and may be regarded as being
the maximum capacity of the road. As the density increased
past this critical point, there was a big reduction in the
capacity of the road and the traffic flow. The road capacity
continued to decrease as the density increased and a traffic
jam could easily occur. The region below the critical density
corresponds to the “free-flow region” where the traffic flow is
less than the road capacity, whereas the region above the
critical density corresponds to the “congested flow region”
in which the flow is limited by the road capacity. It can be
seen that there is usually a “low-speed” value of density and
a “high-speed” value of density corresponding to a given
capacity. The capacity of a road is therefore not a constant,
but is a function of traffic speed and traffic density. Because
the capacity of each road in a traffic network is flow-dependent, the capacity of the road network must also be flowdependent. The properties of networks with flow-dependent
capacities have been studied by a number of authors (e.g.
Shahrokhi and Matula, 1990; Burkhard et al. , 1993; Fleischer
and Tardos, 1998; Baumann and Köhler, 2007)