By John Allen

Triggering a roadway vehicle detector to get a green light can be difficult for a bicyclist, but it is the safe and cooperative option. Designed to detect metal near their sensors, many traffic light vehicle detectors in Massachusetts have a hard time detecting a bicycle. Ride leaders may wish to check vehicle detectors on their routes, and avoid them if they cannot easily be made to work. But there are ways to try to get them to cooperate, as detailed below.

Scoping out the situation

Inductive loop (highlighted, left). Video camera (top right). "Wait here" marking (lower right)

• If you ride through an intersection repeatedly, you can determine whether, and how, vehicle detection works there.
• Wire cuts in the road surface show where detector loops are, unless the street was repaved after they were installed. Alternatively, there may be a video camera overhead, pointing toward the traffic lane.
• Signs and pavement markings may indicate where to wait.
• Only the more lightly used entrances to an intersection may have vehicle detectors. The more heavily used entrances may always have the green light unless cross traffic triggers a vehicle detector.
• Signals may or may not trigger at opposite entrances to the intersection at the same time. This is especially common with left-turn arrows. On familiar routes, you may get to know whether a vehicle waiting at the opposite entrance will get you a green light.

Vehicle detector types

• A dipole loop -- a plain rectangle, octagon or circle -- is the most troublesome kind. It responds over a wide area outside its perimeter, and so the sensitivity must be low to prevent it from responding to a vehicle in another lane. It is most sensitive directly over the wires.
• A quadrupole loop, a more bike-friendly design, has a wire down the middle, and is one of the few instances of special infrastructure for cyclists which encourage lane control (positioning of the bike in the travel lane to discourage unsafe overtaking). A quadrupole loop usually works for bicyclists who place their wheels on its center line.
• A vehicle detector with diagonal wires inside a polygon or circle is sensitive over its entire area.

Making a vehicle detector work

Turning the front wheel to the right across a dipole loop, and making a “do not pass” hand signal.

• An inductive loop vehicle detector responds to the metal of wheel rims and/or to bead wires in tires. Except with detectors which have diagonal wire cuts, position the bike’s wheels directly over the loop wires and in line with them. Wheels resting merely an inch to the side are less likely to be detected. You may want to lift and reposition the rear wheel.
• Skinny tires and larger wheels bring the rim closer to the ground, and are more likely to trigger the vehicle detector. The metal hoops formed by the rims look like solid discs to the long-wavelength radio signals from the detector.  The thickness, weight or magnetic properties of the rims don’t matter.
• Place your wheels over the wire cut where the filler material in the pavement is continuous. An interrupted wire cut is from an older loop that broke and was replaced.
• To avoid a right hook threat, it is a good idea to place your rear wheel over the wire at the left side of a dipole loop. Ride to the front of the loop and turn the front wheel sideways. This places the wheels over two sides of the loop, and establishes lane control (happily, you need to trigger the signal only if no motor vehicle will trigger it, so controlling the lane is easy).
• Leaning your bike toward the center of a dipole loop may help, so put down the foot that is inside the loop.  You may need to practice putting down either foot.
• If you run carbon-fiber wheels and Kevlar-bead tires, you might try wrapping wire under the rim tape. Note that you must connect the ends so it forms a loop. Magnets that attach to the frame may be available, but do not work.
• A video detector overhead on a signal mast arm usually does not require you to do anything special, except at night. Then you may have to tilt your bicycle and aim the headlight upward at it.

The continuous wire cut is for the newer loop, the one which works.

Strategies

• If one vehicle is already waiting, your best position is second in line, behind it. You might slow and let a vehicle pass before reaching the intersection to set up this situation.
• If you are first in line and a vehicle behind you is hanging back, you might pull ahead slightly and motion to the driver to pull forward to trigger the vehicle detector.
• If the right-turn signal of a vehicle behind you is flashing, you might move left in the lane to encourage the driver to pull forward. But then if the vehicle leaves, the signal may not trigger. You might have to discourage an actual right turn with a right-handed "don't pass" signal. Too bad! But other than that, you are not preventing anyone from advancing -- the light is red.
• Once the signal triggers on a vehicle ahead of you, ride over wire cuts to try to hold the light green longer.
• The more wheels are over the wires, the more likely the signal will trip. A group must work together to make this happen. Ride leaders may wish to explain this strategy beforehand.

The group of cyclists has been waiting...and could have triggered the signal before the car arrived. Right-hook risk too!

Cautions

• If there is another lane to your right, you could get stranded to the left of moving traffic if you don't trigger the vehicle detector. This is one important reason to know the signals on your route.
• If a line of vehicles is waiting, the usual cautions about filtering forward apply. Do not pass large trucks on the right!
• Be prepared to accelerate and cross the intersection quickly. A vehicle detector will extend the green time only for vehicles that it senses.

The above information can help cyclists safely utilize vehicle detectors on our roadways. For more technical details about vehicle detectors, see the article by Steve Goodridge here.

The triple photo is from CyclingSavvy.org.  The author wishes to thank Steve Goodridge for permission to use the illustration of vehicle-detector types.