6.8.6 Stop-line Loop Detection
A detection system that employs stop-line presence detection on a lane-by-lane basis is the most common method used in Australia. This has been a result of the development of the SCATS wide‑area traffic control system (Lowrie 1982, 1992, 1996, 2001).
Stop-line loops require greater sensitivity as slow-moving or stopped vehicles must be detected. The location of the loop in relation to the stop line must ensure that the normal stopping position of the first vehicle is in the detection zone.
Figure 6.16 shows location and layout of 4.5 m and 11 m loops, and loop configuration. The lateral dimension of the loop should be derived from Table 6.9 for each situation as indicated in Figure 6.16.
The 11 m detector may be used at locations where a shared or exclusive right turn lane permits filtering and a right turn phase is also provided (Figure 6.16). In practice, the 11 m detector is split into two 4.5 m sections (designated approach and departure loops) that act together in some conditions and separately in others. The longitudinal position of an 11 m detector may vary depending upon intersection geometry.
|Spacing from each outer wire of loop to lane edge|
Note: d1 and d2 in Figure 6.16.
The transverse spacing in Table 6.9 between the outer conductor of the loop and the lane boundary has been chosen to minimise:
- unwanted detection of vehicles in the adjacent lane (over-counting)
- the number of undetected vehicles (especially two-wheeled vehicles) which do not travel through the loop’s zone of influence (undercounting)
- the consequences of missing a vehicle (such as a two-wheeled vehicle) and therefore not servicing that vehicle with a green light.
The optimum spacing between detectors in adjacent lanes is theoretically achieved when over‑counting errors are equal to undercounting errors. A spacing of 1.4 m has been found to provide a good compromise.
The gap between the two sections comprising each 4.5 m loop must be kept to a minimum to give good longitudinal response for all classes of vehicle. However, the smaller this gap, the more the overall sensitivity is reduced. In practice, the dimensions in Figure 6.16 represent a good compromise between the various factors.