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Inductive-loop traffic detector systems operate by sensing disturbances to the electromagnetic field over a coil of wire built into the roadway (Figure 1). When a conductive object (typically made of metal) enters the area over the wire loop, the magnetic field generated by alternating electrical current in the signal detector circuit induces weak electrical currents in the conductive object. (The AC frequency may be between 10,000 and 200,000 Hz, typically around 20,000 - 30,000 Hz.) The electrical currents induced in the object generate their own magnetic field that works in opposition to the magnetic field generated by the sensor coil (due to Lenz's Law). This opposition changes the resonant frequency of the sensor circuit by reducing the effective inductance of the sensor coil. This change in resonant frequency (an increase in frequency as the inductance decreases) is detected by the circuit instrumentation in the signal controller cabinet, which then tells the signal control electronics that a vehicle is present.

A number of variables affect the degree to which the introduction of a conductive object will change the effective inductance of the sensor loop. These variables include:

The size, shape, and conductivity of the object
The 3-D orientation of the object with respect to the wires in the loop
The 3-D position of the object over the loop
The size and shape of the sensor loop
The nominal operating frequency of the circuit

The combination of these variables create the potential for public confusion about the feasibility and reliability of bicycle detection via inductive loop sensors. Experiments that fail to control for some of these variables often create unreliable results, and have sometimes frustrated efforts to select reliable detection systems for cyclists by leading the engineers and facilities designers to false conclusions. Since bicycles are small vehicles and have less conductive material in them than do automobiles, they are harder to detect with inductive loops. Often the sensor loop is very large or the detector circuitry is not sensitive enough to detect the slight inductance decrease caused by the bicycle. The bicycle may not be aligned for maximum effect, or the loop may be shaped (as in Figure 1) such that large vehicles in adjacent lanes may be detected at the level of circuit sensitivity required to register a bike. Since the time and money invested by most states and municipalities toward bicycle transportation issues is very limited, traffic signal engineers often give up on the problem of bicycle detection before it is fully understood. However, this article will show that careful application of the operational theory allows optimization of inductive loop sensing systems for reliable detection of conductive (including aluminum, steel, and titanium) bicycle rims, without false detections caused by adjacent traffic.

There is a common misconception that an object must be ferrous (include iron) to activate a traffic signal loop sensor, or that a ferrous object will perform better. This misconception is fed by the observation that steel cars are detected by standard loop detectors but small aluminum bicycles often, but not always, are not. The belief is rooted in the observation that placing a ferrous core into the center of an inductor coil (such as inside an electromagnet or transformer) affects the inductance of the coil. But in such ferrous-core coil applications, the inductance of the coil is increased by the ferromagnetic effect of the iron, while the typical inductive-loop signal sensors used for traffic signal actuation require the vehicle to cause a decrease in inductance. The iron cores used in typical power inductor applications provide an inductance boost for low frequencies such as 60 Hz. But at higher frequencies, the inductive coupling of eddy currents into the iron core often defeats the inductance boost of the iron. Ferrous inductor components manufactured for high-frequency circuits require a special form of powdered iron called "ferrite" which is designed to minimize its conductivity (especially large-loop conductivity) and thus minimize eddy currents. The steel in cars, by contrast, is highly conductive. Given the high frequencies at which signal detectors operate and the large conductive silhouette of the car, any effect the iron's properties might have to increase the inductance of the coil are overpowered by the induced electrical eddy currents in the vehicle which serve to reduce the inductance of the coil. There are some rare cases where a steel-belted radial with poor loop conductivity positioned in the center of a traffic signal loop can create a net increase in loop inductance, but most traffic signal sensor circuits will either ignore this increase or treat it as an error condition. In short, it is purely the size and net conductivity of an automobile that makes it easier to detect than a bicycle.

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Also, there is a sign that reads "no pedestrian crossing", or "pedestrians prohibited". That is a nice way to get people to walk. I used to make a point on jogging through that intersection in clear violation with the local laws. Good luck in court trying to give me a ticket for that.

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