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Making Vehicle Detectors Work

by Fred Oswald, PE, Bicycling Safety Instructor

This article describes magnetic induction vehicle detector loops and how to work them from a bicycle or motorcycle.

Many traffic lights are controlled by “vehicle detectors”.  Typically, these detectors work through electromagnetic induction, using loops of wire buried in the pavement to sense the presence of a vehicle (actually any conductor) by the disturbance it creates in the magnetic field of the loop.  Your bike need not be made from steel or other magnetic material; it needs only to be an electrical conductor.  Aluminum wheel rims are excellent for triggering vehicle detectors.

Bicycles have much less metal than motor vehicles, thus it can be somewhat more difficult to make loop detectors sense them.  But a knowledgeable cyclist can make many of them work.  The trick is to recognize the wire cuts (typically by the grey filler) and place your wheels in the “sweet spot” -- directly over a loop wire.

There are several patterns of loops.  Most older loops are dipoles (simple rectangles).  Many newer types use a quadrupole (double) loop that incorporates a third wire running down the center of the rectangle.  The quadrupole type is generally better able to detect bicycles and it has a much wider “sweet spot” right in the middle.  However, even most dipole loops can be made to work if they are adjusted carefully and if cyclists know where to stop.

See Figure 1 below for depictions of these loops.  The dashed lines indicate the magnetic field from the loops.  The field forms a flattened half-cylinder above the pavement.  The field at the top of the cylinder will detect the floor pan of a car.  A bike has little horizontal metal but if the wheels are placed directly over a wire where the field is nearly vertical, the aluminum rims can trigger the circuit.

Since a bicycle has very little metal, cyclists must generally stop directly over a wire loop (sometimes within 1 cm) to “trigger” the detector.  This causes problems.  (1) Few people know how these detectors work.  (Auto drivers do not have to know.)  (2) If cyclists cannot see the wire cuts, they cannot see where to place their wheels.  (3) A few detectors are set incorrectly (not sensitive enough) even if the bicycle is directly over a wire.

Vehicle detector loops should be correctly adjusted, tested and marked as specified in Section 9C.05 of the Manual of Uniform Vehicle Control Devices.  (See Fig. 2.)

Using Vehicle Detectors

In Fig. 3 below, the cyclist is just starting out after getting a green arrow.  She had stopped over the loop wire in the middle of the left turn lane.  The wire cut is visible as a faint grey line (marked on photo with yellow arrows) just right of a darker pavement seam.  This loop is a quadrupole, one of the better types for detecting a bicycle.  Unfortunately, the road has been re-paved and the loops are now hidden under asphalt so they are no longer visible.  (Location, Bagley Rd. westbound at Big Creek Pkwy. Middleburg Hts., OH)

Using detector

In Fig 4, the cyclist has just gotten the green from a dipole loop.  He had stopped over one of the side wires.  Although a dipole loop may not be as sensitive to bicycles as the quadrupole loop shown above, you CAN make many of them work if you stop directly over the wire cut.

This intersection has two sets of cut marks that make finding the correct spot harder.  Hint: The newer cuts (covered by a wide streak of gray filler) are the working ones.  This loop was also hidden when the road was repaved.  (Location, Middlebrook Rd at Bagley Rd, Middleburg Hts., OH)

There is also a wire on the other side of the loop at the right side of the lane.  This author would normally choose the left wire rather than the right as did the cyclist shown here.  Can you guess why? [1]

If you use non-metallic rims you should add a loop of wire to actuate vehicle detectors.  The wire ends should be soldered together to complete the loop.  Hold the wire in place with rim tape.

Video Detectors

Video technology can also be used to sense vehicles at traffic lights.  A small video camera can be seen above the traffic lights in the photo below.  With the proper sensing algorithm, this should detect either a car or a cyclist waiting for the light.  (Location, Euclid Ave. at E 18th St., Cleveland)

Video camera detector

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