Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 181 of 265
22.2 Green-Arrow Restrictions on Vehicle Actuated Mode
UK requirements dictate that green-arrows do not lose right of way without their
associated 3-aspect traffic phase showing the amber leaving period. This places
restrictions on the way the controller is allowed to operate.
22.2.1 Indicative Green-Arrow Stage Restrictions
If phase C is an indicative green-arrow (and phase E is a normal 3-aspect traffic
signal), the move from stage 2 to stage 1 should be configured to go via an all-red
stage (in this case stage 0) as described in section 6.2.4.2.
This ensures that when the green arrow is extinguished, it is followed by the amber
leaving period of its associated phase, in this case phase B.
Otherwise, on the move from stage 2 back to stage 1, the green-arrow would just
switch off while its associated phase remains at green.
Due to this restriction, it is commonly required that stage 2 is prevented unless there
is a demand for the side road, in this case either phases D or E so the controller
always moves from stage 2 to stage 3. The move may also be restricted unless
there is a demand for phase C as well – see section 22.2.2 below.
Thus the controller remains in stage 1 even if demands exist for phase C and gaps
appear in the traffic on phase A that would normally allow the controller to ‘gap
change’ to stage 2. However, since the vehicles demanding phase C should be able
to turn across phase A through these gaps, the demand for phase C should clear
naturally.
This is just one of the many ways that Special Conditioning can be used to modify
the controller’s normal stage change algorithm.
22.2.2 Filter Green-Arrow Stage Restrictions
If phase E is a filter green-arrow (regardless of how phase C is configured), the
move from stage 2 to stage 1 (and all other moves out of stage 2) should be
configured to go via stage 3 as described in section 6.2.4.1.
This ensures that the appearance of the green filter arrow is followed by its
associated phase; in this case phase D, appearing at green followed sometime later
by phase D’s amber leaving period.
When phase D appears at green, the green-arrow is normally extinguished (since
the full green signal on phase D’s signals naturally allows the traffic to turn left).
Alternatively, the green-arrow can be configured to remain illuminated through stage
3 until phase D loses right of way.