This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Bob Shanteau <firstname.lastname@example.org>
A traffic loop detects metal objects such as cars and bicycles based on
the change in inductance that they induce in the loop. The loop is an
inductor in an LC circuit that is tuned to resonate at a certain
frequency. A metal plate over the loop (like a car) causes the magnetic
flux to be shorted, reducing the inductance of the loop. This causes a
change in resonant frequency, which is detected and sent to the signal
controller. One of the ways of testing a loop is to create a loop about
2 feet in diameter with several turns of wire (connecting the ends) and
placing the test wire in the middle of the traffic loop. The test wire
should cause a dectection, if all is working.
The same effect is seen with a vertical piece of metal, such as a
bicycle, but is weaker. Because aluminum conducts electricity quite
well, aluminum rims help. Steel rims are OK. Non-metal rims cannot be
picked up at all. A bicycle with aluminum rims will cause about 1/100
the change in inductance of a car.
It is always possible to set a detector's sensitivity to pick up a
bicycle. The trade-off is in longer detection times and the possibility
of false detections from vehicles in adjacent lanes. Most people who set
signal detectors use the lowest sensitivity setting that will pick up
I advocate using the highest setting that will avoid picking up vehicles
in adjacent lanes. Digital circuits used in modern detectors can use
high sensitivity settings without unacceptable increases in detection
times. Unfortunately, there are still a lot of old detectors out there,
and most people who work on signals use principles based on the
performance characteristics of old detectors.
In any case, bicyclists should, as a general rule, place their wheels
over one of the slots to maximize their chance of being detected. That
is where the magnetic field perpindicular to the wheels is strongest.
Bouncing the bike or moving it back and forth does no good. If you have
a metal frame, another tactic that may work is to lay the bicycle down
horizontally inside the loop until the light turns green.
Advancements are under way that may make traffic loops obsolete some
day. In particular, radar, infrared and sound detectors have been
introduced. Systems based on video cameras are especially promising.
Such systems can easily detect bicycles. Such a system may even be able
to detect pedestrians some day.
Bob Shanteau, PhD. PE
Registered Traffic Engineer