Specifications

ST900 Family General Handbook

667/HB/32900/000 Issue 11 Page 261 of 265

STAGE 4STAGE 3STAGE 2STAGE 1

Stage 1 allows the traffic from the north and south to flow while stage 2 allows the

heavier traffic from the north to also turn right unopposed by the other traffic.

When the pedestrian phase G has finished its minimum green period in stage 2, it

can be terminated and the left-turn green filter arrow F can be given right of way. If

there were no demand for the pedestrian phase G, special conditioning would

prevent stage 2 and the controller would move to stage 3 instead.

When the flows on phases A and B finish, the east and west traffic is allowed to flow

and the left-turn green filter arrow is switched off when its associated three aspect

traffic signal phase E appears at green.

If the traffic on phases A and B finishes before the pedestrian phase G has run its

minimum green time, the controller would move directly from stage 2 to stage 4

when phase G’s minimum green time expires and the green filter arrow would not

appear.

Ripple changing helps this junction as it allows the controller to start to move to

stage 3 when phase G finishes, but if a gap appears on phases A and B, the

controller can decide to move to stage 4 instead.

However, since phase F does not actually appear in stage 4, the controller would

refuse to ripple change to stage 4. Instead, it would wait until phase F appears

before it would consider moving to stage 4 and terminating phases A and B.

Put another way, once the controller has decided to go to stage 3, phases A and B

cannot be terminated until phase F appears, therefore delaying the appearance of

phases C and E.

Note that the minimum green time for such a filter arrow is normally set to zero so

that it does hold the controller in stage 3. The phase automatically remains at green

for several seconds since it is not extinguished when stage 3 terminates, but when

its associated phase gains right of way. Also note that phase F should be configured

as fixed in stage 3 to ensure that at least one fixed phase gains right of way on that

move. Therefore the controller will always wait for phase F to appear even if the

demand for it and stage 3 was removed.

One way round the problem is to allow the left-turn green filter arrow phase F to

appear with phase E in stage 4, i.e. so it does not terminate when phase E appears

at green. However, this may mean that this green arrow at this particular junction

does not operate the same as other green arrows at other junctions and thus may

confuse the drivers.