Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 158 of 265
ensure that the LRV clears the junction. This is again achieved using the special
conditioning facility.
16.8 Request Delays
There are two delay times that can be configured to allow for the travelling time of
the LRV from the request detector to the junction. These can be seen on the general
timers diagram Figure 14 as well as Figure 29.
The first delay covers the time that is not needed to process the priority request.
This is normally due to the detector not being in the ideal position (for instance due
to an intervening stop). During the first delay, the controller takes no action on the
demand.
The second delay, which forms the later part of the overall delay before the priority
vehicle actually reaches the junction, allows the controller to be prepared to give
instant service to the vehicle. This is usually done by using special conditioning to
prevent any stage moves except to a stage which will serve the requested phase.
This ensures that the inter-greens caused by the stage change and then the
minimum green times of the phases in the new stage do not delay the required
stage move.
There are 8 sets of (1st and 2nd) delay periods (0 - 7) these are normally associated
with the respective priority units 0 to 7.
However, it is possible to allocate two or more sets of delays (2 or more inputs) to
one priority unit in special conditioning. See Figure 30 below.
REQUEST 2
NORMAL ROUTE
FOR PRIORITY
INPUT
REQUEST 1
SECOND
PRIORITY
DELAY
PRIORITY
UNIT 1
SECOND
PRIORITY
DELAY
FIRST
PRIORITY
DELAY
PRIORITY
UNIT 0
FIRST
PRIORITY
DELAY
SPECIAL
CONDITIONING
DELAYS ASSOCIATED WITH UNIT 0
DELAYS ASSOCIATED WITH UNIT 1
END PRIORITY
UNITS
OR
Figure 30 – Two Delays Set For One Priority Unit