Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 255 of 265
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The ‘ripple change’ facility optimises stage changes at large busy intersections that
have phases that overlap into two or more stages.
39.1 What is a Ripple Change?
The ripple change facility is best explained by an example.
Consider the following junction:
A
B
Stage 1
C
B
Stage 2
C
D
Stage 3
E
F
Stage 4
Figure 49 – Ripple Change Example
A traffic controller running in VA mode should move as follows when the extensions
cease (either due to a ‘gap change’ or a ‘max change’) on certain phases. Also see
the description of VA mode in section 22.
It should move from stage 1 to stage 2 when the extensions on phase A cease.
It should move from stage 2 to stage 3 when the extensions on phase B cease.
Alternatively, it should move straight from stage 1 to stage 3 if the extensions on
phase B cease before (or at the same time as) the extensions on phase A cease.
In effect, the controller should leave stage 1 and move to either stage 2 or stage 3
depending whether a gap appears on phase A or phase B first.