Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 113 of 265
8
8
C
C
A
A
L
L
L
L
/
/
C
C
A
A
N
N
C
C
E
E
L
L
There are up to 8 call/cancel units (0 to 7) available which may be used for turning
movements or in conjunction with queue loops or switched facilities.
The input to a unit must remain active for the call period before the output goes
active and inactive for the cancel period before the output returns to the inactive
condition.
If the call/cancel unit were used for a turning movement, the output would be
configured to insert an unlatched demand for the appropriate phase to allow for it to
be cancelled.
Note that each call/cancel unit therefore will only affect the stream in which the
phase resides. No other streams will be affected.
STAGE 1
B
A
STAGE 2
B
A
CC
(Only phases and stages relevant to
call/cancel are shown)
BX BY BZ
AZ AY AX
C
Figure 22 – Call/Cancel Example
In this situation the controller will move from stage 1 to stage 2 if there is a demand
from the call/cancel loop CC for phase C and either a gap appears in the traffic on
phase A or maximum green time of phase A expires. Note that vehicles on phase B
do not affect this decision since phase B has right of way in both stages.
By default, both the call/cancel loop CC and vehicles on phase B would extend
stage 2. However, where the maximum green time for phase B is set much longer
than phase C (typically because there is little turning traffic), this could result in
phase B holding stage 2 at right of way for an excess period, delaying the traffic on
phase A. The controller can be configured to ignore the extensions from phase B
while stage 2 is active so the controller terminates stage 2 when the call/cancel loop
goes inactive or maximum green time of phase C expires. Eventually right of way
returns to stage 1 allowing the traffic of both phase A and B to proceed.
A demand for stage 2 may or may not be dependent on a demand for the side road
and stage 3 (not shown). If it is not, the move from stage 2 back to stage 1 must be
programmed to go via the all-red stage 0 if phase C is an indicative green arrow
rather than a full 3-aspect red / amber / green signal (see section 6.2.4.2).
Also see section 22 for a more complete description of the movements of such a
junction.