Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 257 of 265
controller can decide to move to stage 3 instead. At this point, it terminates phase B,
which allows it to bring on phase D seven seconds later.
Compare this with diagram above and it shows that phase D and stage 3 have been
given right of way much earlier.
In effect, when a gap appears on phase A the controller will terminate phase A and
begin the process of bringing on phase C. Meanwhile, if a gap appears on phase B
the controller will terminate phase B and begin the process of bringing on phase D.
The net result is that the controller can more quickly service the required phases
and thus reduce delays at the junction. Numerically, if a gap on phase B occurred
one second after phase A, phase D and stage 3 would appear six seconds earlier
with a ripple change.
A ripple change will not violate the minimum green time on any phase, nor will it
violate any inter-greens between two phases. However, by its very nature, this
facility will allow the staggered termination and appearance of phases in order to
optimise the flow of traffic through the junction. If the staggered effects are not
desirable, the facility should not be enabled.
39.4 Ripple Change Parallel Stage Streaming Facilities
If the ripple change facility is enabled, it automatically affects all the configured
streams, but runs independently on each stream – a ripple change on one stream
does not affect any other streams.
If it is required that one stream should not ripple change (even though it could) while
another stream should be allowed to ripple change, there are various ways to
prevent the stream ripple changing. For example, in each stage involved in the
possible ripple change, a different fixed dummy phase could be configured.
39.5 Interaction with Other Facilities
This section summarises how the ripple change facility interacts with other facilities
on the controller.
To aid clarity, the descriptions below assume that the controller starts to make a
normal stage change from stage 1 to stage 2 and during that stage change, the
controller ripple changes to stage 3. However, it should be noted that the controller
could ripple change across any stages and not necessarily consecutive stages.
39.5.1 Modes
If enabled on a controller, the ripple change will be available in all modes, however
it will only really have any impact in VA, bus priority and emergency vehicle modes.
The other modes tend to demand particular stages and thus do not normally allow
the controller flexibility to change the stage movement part way through since the
demand is not cleared until the stage gains right of way.