L
OOP
D
ETECTOR
H
ANDBOOK
667/HE/20663/000
Issue 13
Page 11
1)
Refer to the STS drawing and mark the locations ax, bx, cz 1.1 metres
towards the top line for the loop approach distances X, Y, Z shown on
the STS drawing. (In Figure 3
loop ‘a’ is for a three lane configuration,
‘b’ is a two lane and ‘c’ is a one lane configuration loop.)
2)
From the point ‘ax’ in loop ‘a’ mark the point ‘aa’ 1 (one) metre from the
centre line and at right angles to the kerb through point ‘ax’.
3)
Measure the linear distance between points ‘ax’ and ‘aa’ and divide this
distance by the number of traffic lanes, which in this case is three.
Assuming S1 is 1/3 of total distance between ‘ax’ and ‘aa’, mark points
‘ax’ and ‘aa’. Mark points ‘ay’ and ‘az’ at distances D1 as shown.
4)
At measurements D1 from points from ‘ay and ‘aa’ mark points ‘ay1’,
‘aa1’ as shown.
5)
Loops b and c are marked out using identical techniques, but for two
and one lane respectively.
6)
When all points referred have been marked, the points ‘xx’ can be
marked 1.1 metres from the marked points as illustrated.
7)
When the marking of loop structure points have been completed, the
points can be joined together if necessary for slot cutting purposes.
2.3.7
UD Loops
– SDE/SA, SCOOT, Count and Queue
Figure 4 illustrates a three-lane VA approach system D with diamond loops.
The UD loop overlays the actuating loop by 1/3 of the loop diagonal as shown.
Diamond UD loops can be used with Plessey ST, Microsense detectors and
Sarasota units.
Figure 5 illustrates a three-lane chevron loop approach.
This configuration can be used with Microsense, Plessey ST and Sarasota
detector units.
Note: In a two-lane approach the angle of the UD loops should be reversed to
line up with the Y and Z loops.
Figure 6 illustrates a SCOOT detector site with two carriageway loops.
The length of SCOOT Loops in the direction of travel is fixed as 2 m. This is a
compromise between a loop being short enough to detect gaps in the traffic
stream and long enough to detect stationary vehicles in a queue.
The width of the loop is dependent on site conditions and should not be less
than 1 m, or more than two lanes wide. Factors affecting the width are:
a)
The offside edge of the loop should be approximately 1 m from the
centre line of the road to reduce the probability of under-counting and
over-counting due to vehicles straddling the centre line. The centre line
is taken to be the line least occupied by vehicles in either direction and
may not coincide with the marked centre line.
b)
The loop should be sited where the risk of parking over it is minimal.
Experience has shown that double yellow lines are insufficient
safeguard and unless other physical deterrents to parking exist (e.g.
guard-rail) it is recommended the nearside edge of the loop to be
located 2 m from the kerb.
