Detection of Bicycles at Intersections:
A Question of
Many intersections have signals controlled by systems that
detect vehicles waiting for a green light. The most common systems
use loops of wire embedded in the pavement. These loops function
like metal detectors, identifying the presence of vehicles because
their bodies conduct electricity, disrupting the magnetic field
around the coils.
Unfortunately for bicyclists and many motorcyclists, some
jurisdictions do not consider smaller vehicles when installing and
adjusting traffic light sensors. Some traffic engineers do not
think of smaller vehicles, others mistakenly believe that
inductive loops cannot detect bicycles or small motorcycles.
Bicyclists and motorcyclists are left sitting at red lights
through mutliple cycles of the lights, unable to trigger a green
light for their signal. Sometimes this is an inconvenience -- if a
cyclist is in the right lane, it is often possible to walk over to
a pedestrian button to trigger a green light.
But consider the plight of a cyclist trying to make a left turn
on a multi-lane street:
After determining that a sensor has failed to detect his
vehicle, the cyclist does not have a pedestrian button available
to trigger the light. One option is to treat the signal as
defective, wait for an opening in traffic, and carefully proceed
against the red light. The other is to walk to the pedestrian
button, which requires crossing one or more lanes of traffic on
foot, without the benefit of a protected pedestrian signal phase.
Also, the pedestrian button does not trigger a left-turn signal,
only a pedestrian crossing, so the bicyclist or motorcyclist must
make two pedestrian crossings in place of a single left turn.
A Known Problem With Established Solutions
Fortunately, this situation has been known for many years, and
solutions are available that have been extensively reviewed by
traffic engineers, and successfully implemented in many
jurisdictions. A cyclist who must deal with unsresponsive signals
does not need to invent a solution.
Adjust Sensors to Detect Bicycles
Some traffic engineers believe, incorrectly, that sensors
cannot detect bicycles. The profession has even developed myths to
justify this perception, e.g. sensors require ferromagnetic
materials, or large masses of material, in order to function. But
the truth is, the sensors detect any electrically conductive
material that disrupts their fields: even a single aluminum-rimmed
bicycle wheel can trigger many properly-adjusted sensors.
The sensors should be adjusted to be as sensitive as possible
without excessive false triggering by traffic in adjacent lanes.
Spurious triggering can waste drivers' time by adding left-turn
phases when no traffic is actually waiting to turn left. But this
must be weighed against the consequences of trapping cyclists at
non-functional signals -- few traffic engineers would
intentionally risk the lives of cyclists just to save time for
Mark Sensors' "Sweet Spots"
Even when a sensor is properly adjusted, it will not always
detect a bicycle or motorcycle. These vehicles are much smaller
than cars or trucks, so they are detected most reliably if they
stop directly over the most sensitive part of the loop.
When a loop is freshly installed, it is usually possible to see
the pavement cuts for the wire, at least in good lighting. But
different loop designs have different "sweet spots",
pavement cuts are hard to see at night or in heavy rain, and
resurfacing can hide the cuts.
To ensure the most reliable detection of cyclists, the traffic
authority can mark the best place for bikes to wait for a signal.
Different jurisdictions have tried different markings for this
purpose, such as simple "T" or "X" markings on
the loop. But if markings are not consistent from one town to the
next, and if their meaning is not obvious, many cyclists will not
know to stop over the markings. The best approach, then, is a
The Federal Highway Administration's Manual
on Uniform Traffic Control Devices provides standardized
signs, signals, and markings for all sorts of traffic situations,
and this is no exception. MUTCD Section 9B-12 provides two
standard traffic devices for bicycles at signals. Figure 9C-7 is a
standard pavement marking for bicycles at signal detectors, and
R10-22 is a sign that instructs cyclists to wait on the symbol to
request a green light.
Presenting Your Request
In many communities, bicyclists are stereotyped as hippies,
raving leftists, or drivers with one too many DUIs. While there
may be a small kernel of truth to each of those stereotypes, you
do not want your public officials to consider you an
easily-marginalized malcontent. Try to present yourself and your
request calmly and professionally, even though the city may have
unintentionally created an intersection that is likely to cause
accidents. The city probably did not set out to cause accidents,
the defective signal is probably an oversight, or a result of
misinformation the design engineers have received from other
If you are reporting a defective signal by phone, wait until
you have calmed down. Don't yell in their ear while panting from a
hard ride -- it makes you sound like a raving leftist and plays
right into that stereotype. I suggest a simple, straightforward
approach: "Hello, yes, how do I report a defective traffic
signal? I sat in the left turn lane at First and Main for three
cycles of the light and never got a turn arrow." Save any
more details until you're talking to the person who actually
handles this sort of problem, restate the situation, then
mention that you were on a bicycle. This starts the conversation
in terms they understand, a defective signal, rather than starting
off as yet another bicycle complaint.
If you are reporting a problem in person, take the time to
dress as you would for any other visit to a government office.
Sweaty spandex and helmet hair do not encourage a motorist to feel
a sense of community with you -- you want to look like an average
taxpayer making a perfectly reasonable request. And you want to
talk like an average taxpayer making a perfectly reasonable
request. Again, start by discussing the problem with the
intersection, not the fact that you were on a bicycle when you
encountered the problem.
References & More
This article grew out of an exchange of email and letters with
a small city in King County, Washington, discussing a signal of
theirs that is maintained under contract by King County.
At the time, King County staff wrote that their policy was that
bicyclists should ignore the rules of the road and use the
pedestrian buttons to make left turns on busy roads.
Some months after presenting the information above to the
County, along with examples of other local jurisdictions whose
traffic signals are adjusted to detect bicyclists, I received a
letter from the roads engineering department stating that they had
reviewed my information, consulted with other jurisdictions, and
decided to change their policy County-wide. New signals will be
installed with the needs of bicyclists in mind, and existing
signals will be upgraded during regular scheduled maintenance over
the next several years.
As a voter, taxpayer, and bicyclist, I would like to thank King
County for this decision.
About two years after my conversation with King County, the
Washington State Legislature had the wisdom to pass legislation
requiring that, among other things:
- All existing vehicle-activated traffic control signals that
do not currently routinely and reliably detect motorcycles and
bicycles must be adjusted to do so to the extent that the
existing equipment is capable consistent with safe traffic
Where motorcycle and bicycle detection is limited to
certain areas other than immediately before the stop line or
crosswalk in the center of a lane at an existing
vehicle-activated traffic control signal, those detection areas
must be clearly marked on the pavement at left turn lanes,
through lanes, and limited right turn lanes. These detection
areas must also be marked to allow a bicyclist to leave a bicycle
lane to enter a detection area, if necessary, to cross an
intersection. Pavement markings must be consistent with the
standards described in the state of Washington's "Manual on
Uniform Traffic Control Devices for Streets and Highways"
obtainable from the department of transportation.
With this legislation, it should no longer be necessary for
cyclists in Washington State to reinvent the wheel, so to speak,
when convincing local jurisdictions to maintain their traffic
signals for all legal traffic.
- This page written by Josh
Putnam. Please feel free to email questions, comments,
corrections, suggestions, etc.