In the public debate, congestion is mainly seen as one phenomenon,
neglecting the different kinds and different causes of this loss in transport
service quality. But to find adequate and efficient solutions to the problems
associated with congestion, it is necessary to differentiate between the various
types and the various reasons. This could be:
− A reduction in road capacity caused by an unplanned event, for
example, an accident with wrecks blocking a lane;
− A planned reduction in capacity due to construction or maintenance
of the lane;
− A traffic demand higher than the maximum flow capacity.
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The last point seems to be the most interesting one because of its complex
formation process. Also, this case is the most often-cited reason for
congestion, linked with political demands for more roads. Overload congestion
means that the amount of vehicles moving in a certain direction is higher than a
certain part of the road can carry. When, at a certain point of the road, fewer
vehicles can pass through than want to pass, a queue is formed, growing rapidly
in the direction of the origin of the vehicle flow. A typical case is when the
number of lanes is reduced at a certain point or when several roads or ramps
lead to a part of the network with less capacity than the added vehicle flow
moving towards it.