Density
It is obvious that measurements of traffic density cannot be easily obtained directly in the field with conventional detectors. One way to obtain a single estimate of density is to consider an aerial photograph of a segment of roadway and to count the number of vehicle on one lane. Besides aerial photographs, other techniques that have been used for estimating density include calculation of density from flow and speed measurements, calculation of density from input and output flow counts and measurement of occupancy.
Occupancy is the percent of time that a traffic detector (i.e., an inductive loop,
microwave or video detector) is occupied by vehicles. For a specific time interval,
T, occupancy would be the sum of times vehicles occupy the detector, ti, divided by
T. For each individual vehicle the vehicle's length, Li, and its traveling speed,
vi, as well as the length of the detection zone, d, determine the time spent over
the detector
(See Figure 5 on page 21).
Equation 13
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