Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 10 Page 105 of 265
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7.1 General Description
Demands may be inserted for phases or stages, but extensions may only be
inserted for phases.
Demands will only be acted upon when the phase or stage is not at right of way and
request (i.e. demand) that the phase / stage gains right of way.
Extensions will only be acted upon when the relevant phase is at right of way and
will request that the phase remains at right of way for longer.
7.2 Types of Demands
7.2.1 Latched
The demand remains active until the phase or stage is served, i.e. gains right of
way.
Thus, an activation of a detector (e.g. when a vehicle passes over an inductive loop
or when a pedestrian push-button is pressed) is ‘latched’ so that it keeps requesting
(i.e. demanding) the phase until the phase eventually gains right of way.
7.2.2 Unlatched
The demand is cleared if the demand condition ceases before the phase or stage is
served.
Any maximum green timers started by an unlatched demand will be reset if all of the
opposing demands are cleared.
7.3 Origins of Demands and Extensions
7.3.1 On-Street Detection Equipment
This refers to the approach detectors at the intersection.
These are normally inductive loops cut into the road’s surface or above ground
detectors mounted on the top of signal poles. Both detect traffic moving towards the
signals.
An activation of such a detector normally inserts a ‘latched demand’ for the phase
that gives right of way to the detected vehicle. When the phase gains right of way,
further activations of these detectors normally extend the green period.