CHAPTER 7 CHANNELS-ALERTS-ZONES 81
Figure 7.25 – Dynamic Density Filter
Dynamic Density allows the SS200 to generate outputs based on trac eciency. e
three lter elements (Headway, % Utilization, and Tuning Factor) are combined with the
size of the zone and the average speed of qualied detections to create a detection density
threshold that changes with the uctuations in detected trac.
Headway refers to the time separation of the front edge of two consecutive vehicles travel-
ing in a given lane. e headway metric is inversely proportional to the ow rate of a given
lane and is widely used as a measure of how eciently trac is owing in a given lane.
A typical headway value for heavily traveled lanes is two seconds, which corresponds to a
ow rate of 1800 vehicles per hour. In the Dynamic Density settings, the minimum head-
way value is 0.1 seconds and the maximum headway value is 10.0 seconds.
% Utilization refers to the ratio of observed ow rate to ideal ow rate. It is a measure used
to describe trac eciency in terms of the ideal eciency. In the Dynamic Density set-
tings, the minimum utilization value is one percent and the maximum utilization value is
100 percent.
Note
Consider a case where a lane is operating at the lower bounds of acceptable eciency
when only 75 percent of it’s ideal flow rate was realized. If the ideal flow rate were
1800 vehicles per hour, then the lane is being used adequately at a flow rate of 0.75 *
1800 vehicles/hour = 1350 vehicles/hour (or a headway of 2.67 seconds).
e Tuning Factor is a general-purpose scale factor used to adjust observed results to rep-
resent actual road conditions. e tuning factor resides in the numerator of the Dynamic
Density equation. e initial recommendation for the tuning factor value is to set it equal
to the number of lanes being observed by the sensor. e minimum tuning value is 0.1 and
the maximum tuning value is 10.0.
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