Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 173 of 265
During morning peaks, traffic flowing into the city centre (C-B-A) is required to meet
a succession of signals at green (a ‘green wave’). Typically, free flowing morning
peak traffic takes 20 seconds to travel from ‘C’ to ‘B’ and 15 seconds to travel from
‘B’ to ‘A’. The main road lights at ‘B’ should therefore change to green 20 seconds
after those at ‘C’, and those at ‘A’ a further 15 seconds later.
A 27 Secs19 Secs
16:40:4616:40:00 16:40:19Plan Start:
CB
CITY
CENTRE
30 Secs
15 Secs
25 Secs
20 Secs
25 Secs
20 Secs
Controller Timings:
(45-second cycle time)
Figure 32 – Simple Linked Installation (Evening Peak – Plan 1)
During evening peaks, traffic flowing out of the city centre (A-B-C) is also required to
meet a succession of signals at green (a ‘green wave’). Typically, free flowing
evening peak traffic takes 19 seconds to travel from ‘A’ to ‘B’ and 27 seconds to
travel from ‘B’ to ‘C’. The main road lights at ‘B’ should therefore change to green 19
seconds after those at ‘A’, and those at ‘C’ a further 27 seconds later.
The required offsets between the controllers can be achieved several ways. These
are described in the following sections:
Section 21.6.2 “Using Plan Offset Times (Base Time CLF Only)”
Section 21.6.3 “Using Different Start Times (Non Base Time CLF Only)”
Section 21.6.4 “Using Different Group Times (Either CLF System)”
The group timers in each controller are fully synchronous and, provided the
specified cycle time at each intersection is the same, the offset between controllers
at the start of a particular plan is maintained until the next plan is selected.