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The traffic stream includes a combination of driver and vehicle behavior.
The driver or human behavior being non-uniform, traffic stream is also
non-uniform in nature.
It is influenced not only by the individual characteristics of both vehicle and
human but also by the way a group of such units interacts with each other.
Thus a flow of traffic through a street of defined characteristics will vary
both by location and time corresponding to the changes in the human behavior.
The traffic engineer, but for the purpose of planning and design, assumes
that these changes are within certain ranges which can be predicted.
For example, if the maximum permissible speed of a highway is 60 kmph, the
whole traffic stream can be assumed to move on an average speed of 40 kmph
rather than 100 or 20 kmph.
Thus the traffic stream itself is having some parameters on which the
characteristics can be predicted.
The parameters can be mainly classified as : measurements of quantity, which
includes density and flow of traffic and measurements of quality which includes
speed.
The traffic stream parameters can be macroscopic which characterizes the
traffic as a whole or microscopic which studies the behavior of individual
vehicle in the stream with respect to each other.
As far as the macroscopic characteristics are concerned, they can be
grouped as measurement of quantity or quality as described above, i.e. flow,
density, and speed.
While the microscopic characteristics include the measures of separation, i.e.
the headway or separation between vehicles which can be either time or space
headway.
The fundamental stream characteristics are speed, flow, and density and are
discussed below.
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