Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 116 of 265
Two assessors are used, an outer and an inner. The outer is situated 159 metres
from the stop line and the inner 91 metres. The outer assessor has a threshold
velocity of 45mph / 72kph and the inner 35mph / 56kph. Each assessor provides a
fixed extension period normally of 3.5 seconds for vehicles exceeding the threshold
velocity.
91m
159m
Stop
Line
12’
(3.66m)
12’
(3.66m)
XYZ
Standard ‘D’
Detector
Figure 24 – Triple SDE Layout
9.2 Speed Assessment (SA)
Each traffic lane is provided with an assessor situated 151 metres from the stop line.
A vehicle crossing the assessor at a speed above 28mph / 45kph generates a fixed
extension of 5.0 seconds after a delay period that is dependent on the vehicle
speed. The faster the vehicle speed the less the delay period is. During the delay
period a normal ‘gap change’ can occur.
151m
Stop
Line
12’
(3.66m)
XYZ
Standard ‘D’
Detector
Figure 25 – SA Layout
The formula for calculating the delay period is:
Delay =
140
–
5 seconds
V
where ‘V’ is the measured vehicle speed in metres per second.
For example, the delay at 14 m/s (which is 32mph and 50kph) is 5 seconds and the
delay at 20m/s (which is 45mph and 72kph) is 2 seconds. Above 28m/s (which is
62mph and 100kph) the delay period is zero.
9.3 Assessors Available
There are up to 16 assessors (0 to 15) available for use on SDE or SA.