Specifications
ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 115 of 265
9
9
H
H
I
I
G
G
H
H
S
S
P
P
E
E
E
E
D
D
V
V
E
E
H
H
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I
C
C
L
L
E
E
D
D
E
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T
T
E
E
C
C
T
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I
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O
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N
N
To provide this facility, specially positioned vehicle loops are required. These should
be connected to the Controller through the normal serial I/O cards or Intelligent
Detector Backplanes. The SDE/SA facility is fully integrated in the Controller, but if
the SoundMark interface socket is required for test purposes, a separate SDE/SA
processor card may be used.
On roads where it is required to detect high-speed vehicles to ensure safe passage
through an intersection, one of two methods may be used:
SDE (double or triple) where a fixed extension is generated.
SA where a fixed extension is generated after a variable delay dependent on
the vehicle speed.
9.1 Speed Discrimination Equipment (SDE)
Each traffic lane is provided with either one or two speed measurement points,
assessors, situated further from the stop line than the normal detectors, to
implement either double or triple SDE. An assessor consists of two loops (A and B)
with a 12ft leading edge to leading edge spacing. The loop-loop traversal time gives
a measure of the vehicle speed. A vehicle travelling over an assessor above a pre-
set threshold generates a pre-set extension for the approach’s phase green so it
retains ROW until the vehicle reaches the normal point of detection.
9.1.1 Double SDE
This facility is used at intersections where vehicle approach speeds in excess of
35mph / 56kph and less than 45mph / 72kph are normally anticipated.
The standard assessment speed is 30mph / 48kph and the distance of the assessor
is 79 metres from the stop line. A fixed extension period normally of 3.0 seconds is
provided for each vehicle exceeding the threshold velocity.
79m
Stop
Line
12’
(3.66m)
XYZ
Standard ‘D’
Detector
Figure 23 – Double SDE Layout
9.1.2 Triple SDE
This facility is used where vehicle approach speeds in excess of 45mph / 72kph and
less than 65mph / 105kph are normally anticipated.