Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 182 of 265
Due to this restriction, it is commonly required that stage 2 is prevented unless there
is a demand on phase C from the right-turning traffic (and possibly only if there is a
demand for the side road as well – see section 22.2.1 above).
Thus, if there is no demand for phase C, the controller normally moves from stage 1
to stage 3 (omitting stage 2) if there is a demand for the side road, even if it is from
left-turning traffic that could just use the filter green-arrow (in stage 2).
22.2.3 Filter Green-Arrows and the Main Stage Green
If phase E is as filter green-arrow and is configured to switch off when its associated
phase (phase D) gains right of way, it must not be configured in stage 3 otherwise it
will not switch off when required.
Thus, since phase E now only resides in stage 2 and not stage 3, demands for this
phase will demand stage 2 and not stage 3.
Therefore, the detectors on the approach for phase E are normally configured to
demand and extend phase D (and thus stage 3), not phase E (stage 2).
Even though phase E is now no longer demanded, it will automatically appear in
stage 2 if it is configured as a fixed phase.
Alternatively a dummy phase can be configured in stages 2 and 3 that is demanded
and extended by the detectors on phase E’s approach. An unlatched demand can
be configured (using special conditioning) to illuminate the green arrow during stage
2 if the dummy phase appears due to a real street demand.
22.3 Arterial Reversion
Normally, after a phase has gained ROW, if there are no other demands, ROW will
remain on that phase. The arterial reversion facility allows ROW to revert to a
specified stage or phase in the absence of any demands and extensions, subject to
any minimum green periods timing off and any stage movement restrictions.
If a stage is specified, the reversion will always be to that stage, but if a phase is
specified, reversion will be to the next stage in cyclic order in which that phase
appears.
The name of the facility is derived from its normal use, which is for intersections with
a main (or arterial) movement. During quiet periods, especially at night, ROW will
revert to the main movement after a side road has had ROW, so that isolated
vehicles on the main movement will not have to demand ROW and then wait for an
inter-green period from the side road to time off. In this case, a stage would normally
be specified for reversion.
Another use of the facility is on bridges with shuttle working. If the all-red movement
were reverted to, isolated vehicles from either direction would only have to wait for a
red/amber period before gaining ROW. If more than one all-red stage is used with