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The basic assumption of this model is ``A good rule for following another
vehicle at a safe distance is to allow yourself at least the length of a car
between your vehicle and the vehicle ahead for every ten miles per hour of
speed at which you are traveling"
According to Pipe's car-following model, the minimum safe distance headway
increases linearly with speed.
A disadvantage of this model is that at low speeds, the minimum headways
proposed by the theory are considerably less than the corresponding field
measurements.
In this model, the reaction time needed for the following vehicle to perceive
the need to decelerate and apply the brakes is considered.
That is, the time gap between the rear of the leader and the front of the
follower should always be equal to or greater than the reaction time.
Therefore, the minimum time headway is equal to the reaction time (minimum time
gap) and the time required for the lead vehicle to traverse a distance
equivalent to its length.
A disadvantage of this model is that, similar to Pipe's model, there is a wide
difference in the minimum distance headway at low and high speeds.
The General Motors' model is the most popular of the car-following theories
because of the following reasons:
- Agreement with field data; the simulation models developed based on
General motors' car following models shows good correlation to the field data.
- Mathematical relation to macroscopic model; Greenberg's logarithmic
model for speed-density relationship can be derived from General motors car
following model.
In car following models, the motion of individual vehicle is governed by an
equation, which is analogous to the Newton's Laws of motion.
In Newtonian mechanics, acceleration can be regarded as the response of the
particle to stimulus it receives in the form of force which includes
both the external force as well as those arising from the interaction with all
other particles in the system.
This model is the widely used and will be discussed in detail later.
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