Safety Vehicle detector loops cut into the road surface to detect vehicles. These will prevent the equipment from closing/lowering on a vehicle within the aperture.
A quadrupole loops is actually two loops wired in a figure-8 pattern side-by-side, in series with a single wire. The name Quadrulope comes from the sensor having four poles (each loop has two magnetic poles). The two poles are wound in opposite directions, such that whenever the magnetic North is point up out of one loop, it is pointing down into the adjacent loop. This creates a tight channelisation of magnetic flux from one loop over into the other, resuting in maximum sensitivity over the centre of the sensor footprint with much less spillover around the sides. Also, note that the centre sawcut in the loop, which runs parallel to the direction of vehicle travel, has twice as many conductors in it as the edge sawcuts, and the current runs in the same direction for all of the centre-sawcut conductors. This makes the centre sawcut the most sensitive place over which to position motorcycle wheels. The quadrulope loop offers at least four significant advantages over standard loops:
A small quadrulope loop at high sensitivity can usually detect a motorcycle anywhere over the loop, not just over the centre wires. However, detection of a motorcycle does requires that the detector circuit to be set to a more sensitive. It is also recommended that markings be placed in the roadway to identify the centre of the loop to motorocyclists.