The Gravity Model
The gravity model is much like Newton's theory of gravity. The gravity model assumes
that the trips produced at an origin and attracted to a destination are directly
proportional to the total trip productions at the origin and the total attractions at the
destination. The calibrating term or "friction factor" (F) represents the
reluctance or impedance of persons to make trips of various duration or distances. The
general friction factor indicates that as travel times increase, travelers are
increasingly less likely to make trips of such lengths. Calibration of the gravity model
involves adjusting the friction factor.
The socioeconomic adjustment factor is an adjustment factor for individual trip
interchanges. An important consideration in developing the gravity model is
"balancing" productions and attractions. Balancing means that the total
productions and attractions for a study area are equal.
Standard form of gravity model
Where:
Tij = trips produced at I and attracted at j
Pi = total trip production at I
Aj = total trip attraction at j
F ij = a calibration term for interchange ij, (friction factor) or travel time factor
( F ij =C/tijn )
C= calibration factor for the friction factor
Kij = a socioeconomic adjustment factor for interchange ij
i = origin zone
n = number of zones
Before the gravity model can be used for prediction of future travel demand, it must be
calibrated. Calibration is accomplished by adjusting the various factors within the
gravity model until the model can duplicate a known base years trip distribution.
For example, if you knew the trip distribution for the current year, you would adjust the
gravity model so that it resulted in the same trip distribution as was measured for the
current year.
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