6.2Sensor installation for vehicle detection site#

6.2.1Numbering of sensors#

6.2.1.1Intersection sensors#

For numbering sensors at an intersection, refer to the Drafting and Design Presentation Standards Manual (DDPSM) Volume 2 Chapter 2.

6.2.2Inductive loop installation#

Vehicle detection loops are to be located as shown on SD1424 Traffic Signals - Detector loops installation details asphalt pavement and SD1425 Traffic Signals - Detector loops placement details.

Vehicle detection loops installed on the department’s road network are to be installed in asphalt or in concrete pavement. Loops installed in bitumen seals are not covered in this document.

When determining the placement of detector loops, the following points should be considered regarding loop installation:

• a) loop wire needs to be close to the road surface for optimum sensitivity: the minimum distance to surface is 15 mm
• b) loops cannot be installed over any bridges, culverts, stormwater drains, or similar structures, unless there is at least 80 mm of covering pavement
• c) loops cannot be installed closer than 300 mm to any ferrous metal object such as a manhole cover, pipe or reinforcing
• d) the distance between the loop and its detector unit, normally located in the controller housing, shall be as specified by the manufacturer
• e) inductive loops on new pavements shall be pre-formed unless otherwise specified by the Principal
• f) loops installed in new pavements shall be pre-formed, and
• g) loops installed in existing pavements may be formed insitu.

The location and/or size of any detector loop may vary to suit special circumstances, such as geometry (for example, driveway, turning lane, parking area), location of expansion joints or ferrous metal objects.

6.2.2.1Loop installed in asphalt pavement#

For installation in asphalt pavement, the Quadrupole loop takes the form of a 'figure 8' as shown on SD1425 Traffic Signals - Detector loops - placement details. The following are the requirements of rectangular and Quadrupole loops installed in asphalt pavement:

• a) loop cable shall comply with AS/NZS 2276.3 Cables for traffic signal installations, Part 3 Loop cable for vehicle detectors
• b) loop feeder cable shall comply with AS/NZS 2276.2 Cables for traffic signal installations, Part 2 Feeder cable for vehicle detectors.
• c) loop cable shall be continuous, that is, no joints permitted, between Start (S) and Finish (F).
• d) loop leadings (S and F) to each loop shall be twisted together at approximately 1 turn per 100 mm
• e) loop inductance connected across its input terminals lies within range 100 µH to 700 µH and a Q factor in the range of:
• i. 5 to 50 below 60 kHz, and
• ii. 3 to 50 above 60 kHz
• f) loop detector cables and feeder cables shall be jointed in Type 3 pits on the road reserve. Each joint shall be separately insulated and sealed to prevent water ingress
• g) all loop feeders shall be returned to the cable pit in the road reserve (or the median of a minimum 2 m wide) except for right-turn presence loops, or loops in the 2 lanes closest to the median in a 4 or more lane approach, which may be returned to a cable pit in the median if a median post is required
• h) all loop cable ends shall be labelled with Helagrip Markers (HG2 5) or equivalent with Start (S), Finish (F) and numbered as per the design plan
• i) all feeder cables shall be labelled with Helagrip Markers (HG4 9), or equivalent, at each end to show the detector number as per the design plan (for example, 1, 2 and so on)
• j) retaining wedges must be fitted at 300 to 400 mm spacing to ensure loop cable does not move while sealant is applied. The wedge material shall be flexible and water resistant
• k) the loop shall be sealed in the slots using a polyurethane slot sealant or equivalent
• l) the number of turns for loops shall be: Rectangular / Square (4 turns) and Quadrupole (3 turns). Motorway preformed loops have 5 turns, and
• m) for a combined detector cable and feeder cable lengths in excess of manufacturer specified limits, a 1% variation check is required to determine if another loop turn or a signal amplifier is needed.

6.2.2.2Loop installed in concrete pavement#

For installation in concrete pavement, the loop is recommended to be pre-formed and takes the form of a square as shown on SD1424 Traffic Signals - Detector loops installation details asphalt pavement. The following are the requirements of pre-formed square loops installed in concrete pavement:

1. 1.loops shall be pre-formed, square, measuring 2 m by 2 m, supplied with 5 turns (acceptable inductance range: 100 µH 700 µH)
2. 2.pre-formed loops shall be assembled, joined and tested prior to delivery at site. The loops shall be supplied complete with factory assembled lead in cables. Cores shall be continuous: jointing of cores is unacceptable
3. 3.the individual loop cores shall be insulated individually and encapsulated within a flexible sleeve constructed from polypropylene, polyethylene, XLPE or other suitable material
4. 4.lead-in cable shall be a continuation of the loop cables. They shall be insulated individually, twisted together at approximately 4 turns per 100 mm and encapsulated within a flexible sleeve
5. 5.detector loops shall be installed and sealed into slots cut into the top of a concrete sub-base
6. 6.slots for the pre-formed loops shall be cut using conventional loop cutting or milling type equipment.
7. 7.all loop feeders shall be returned to the cable pit in the footpath (or the median with a minimum width of 2 m).
8. 8.loop detector cables and feeder cables shall be jointed in pits.
9. 9.loops in asphalt pavement are to be installed in accordance with MRTS93 Traffic Signals in dense grade with open grade cover.

6.2.2.3Loop cabling standards#

The loop cable shall comply with AS/NZS 2276.3 Cables for traffic signal installations, Part 3 Loop cable for vehicle detectors.

The loop feeder cable shall comply with AS/NZS 2276.2 Cables for traffic signal installations, Part 2 Feeder cable for vehicle detectors.

All loops and feeder cables used must be preapproved by the department.

6.2.2.4Loop feeder cable#

The loop lead-in is the cable between the loop and its nearest pit. The loop feeder cable connects the lead in cable in the pit to the controller. The total length of the cables from the loop to the controller affects the detector sensitivity and should be kept to the minimum and, in any case, within the limits specified by the manufacturer.

6.2.2.5Loop configuration#

6.2.2.5.1Shape of loop#

The requirement to detect all types of vehicles from bicycles and motorcycles to high bed trailer trucks, while avoiding detection of vehicles in adjacent lanes, has led to a variety of loop designs. Detector loop designs used by the department are Rectangular and Quadrupole as shown in SD1424 Traffic Signals – Detector loops installation details asphalt pavement and SD1425 Traffic Signals – Detector loops placement details.

For speed measurement two identical rectangular loops are used. Refer to SD1702 Traffic Signals – Detector loops motorways management placement details for details on the distance between leading edges of loops.

6.2.2.5.2Sizes of loop#

Loop sizes determine the detection zone of the loop.

Loop length depends on the dimension of the target vehicle and expected vehicle speed. Loop lengths used by the department vary from 0.7 m to 2.0 m. In some situations, multiple loops are used in series to give an effective loop length much longer than this. Refer to Table 6.2.2.5 for details of permitted configurations.

Loop width depends on the dimension of the target vehicles to be detected and on the width of the traffic lane. Unless specified otherwise, width of loop varies with the width of the traffic lane. Refer to Note (1) on Table 6.2.2.5 for details.

6.2.2.5.3Number of turns#

The number of turns of the conductor making up the loop determines the loop inductance and sensitivity. All loops should have suitable number of turns to provide a nominal minimum inductance of between 100 to 700 µH.