In principle, congestion is specific to each part of the road network. Adding a car to a
four-lane street has other congestion effects than adding a car to a one-lane street. Here,
we compute marginal congestion costs in a more aggregate way. The basic premise is
that the urban area has homogeneous traffic conditions and can be represented as if it
were a one-link system. Adding one car to the urban traffic then slows down all other
cars using the urban network at that moment.
In the calculation of the marginal congestion costs, the speed-flow relationship is of
crucial importance. It describes how average speed (s) is influenced by traffic flow (q).
Traffic flow is measured in millions of passenger car units (PCU) per hour. PCU are used
instead of the number of vehicles to reflect the difference in congestive effect of the vehi-
cle types considered. Generally, a bus or a truck is assumed to correspond to 2 PCU.
The aggregate speed-flow relationship has to be derived from simulations with a network
model. Such a model is necessary to compute the impact on average speed of a proportional
increase in all trips. Kirwan et al. (1995) conclude that an exponential type of aggregate
congestion function is the most satisfying.
We estimated the parameters of an exponential congestion function for the transport
situation in Brussels, starting from three observation points. The first is the current peak
period situation which is characterized by a traffic flow of 0.5337 million PCU per hour
and an average speed for cars of 38.2 km/h (cf. Region de Bruxelles-Capitale, 1993,
p. 72). In the second observation point, which represents the peak situation in 2005, the
traffic flow is 20% higher than in 1991, and the average speed has fallen to 23.7 km/h
(Region de Bruxelles-Capitale, 1993). Finally, there is the free-flow situation with a
traffic flow equal to zero and an average speed of 50 km/h. The resulting congestion
function expresses the minutes needed to drive 1 km in a certain period as a function of
the million PCU per hour at that moment in the city: