Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 178 of 265
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In VA mode the controller monitors all demands, extensions and maximum green
timers every 200 ms to select a suggested stage in order to satisfy as many
demands as possible without missing any stage containing a phase with a demand.
Section 7 describes the sources of demands, sections 6.5.2 and 6.5.3 describe
green extensions and maximum times and section 5.5 describes the controller’s
stage change algorithm.
If each phase only resides in one stage, each stage will continue until there is a
demand for a phase not at right of way (but also see the Arterial Reversion facility
described in section 22.3).
Even when there is a demand, the stage will continue until all the phases that are at
right of way can be terminated. If a phase still running its minimum green time or a
phase is still being extended (and its maximum green time has not yet expired), the
controller will remain in that stage.
If the same phase is given right of way in two or more stages, the operation of
vehicle actuated mode is made more complicated. See the example below:
22.1 A Vehicle Actuated Example
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STAGE 2STAGE 1
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Figure 33 – Example of VA Stage Changes
The descriptions overleaf describe how the controller applies its next stage
algorithm to the above junction. Initially, all the phases are assumed to be standard
3-aspect traffic phases. Indicative and filter green-arrows complicate the operation
of this junction and so are discussed later in section 22.2.
The decisions in the next stage algorithm are summarised here, assuming no
phases or stages are prevented or deleted and no stage demands exist. See section
5.5 for the detailed descriptions:
Considering each stage in turn
a) Are there any demands for phases in this stage?
b) Do all phases that need to keep right of way appear in this stage?
c) Are all the demanded phases from previously checked stages in this stage?
d) Will additional demanded phases be given right of way?