Originally Posted by
cyccommute
You have to have the wheels directly over the most sensitive part of the loop. For a figure 8 sensor, that means being directly over the middle wire. For the round detectors, that means being over the wire that leads from the loop to the box. In both cases, those wires are a double wire which increases the sensitivity greatly.
You're right about figure-8 loops (technically called quadrupole loops), but not about circular loops. Although it is true that there are "double wires" in the slot leading from the loop to the box, the current in those wires is in opposite directions, resulting in no magnetic field over them. For round and square loops both (which are dipole loops), you need to have your wheels directly over the wire in the loop itself.
The only loop configuration that can detect vertical conducting objects (such as metal bicycle wheels) anywhere within the loop is a diagonal quadropole, where the center wire is on a diagonal. This can be as simple as a round loop with a diagonal cut with the wire wound in a figure-8 or as complex as what Caltrans calls a Type D loop, which is more or less a square loop with two diagonal cuts.