How to Trigger a Vehicle Detector

Triggering a roadway vehicle detector to get a green light can be difficult for a bicyclist — or motorcyclist — but it is the safe and civilized option. Designed to detect metal near their sensors, many traffic light vehicle detectors work reliably only for larger vehicles. Our article on the Idaho Stop for red lights describes what you might do if the light does not change. But let’s try to make it change.

You shouldn’t need to know any of what you are about to read here: signals should just work. But here goes.

Scoping out the situation

If you ride through an intersection repeatedly, you can learn 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. Or there may be a video camera overhead on a signal mast arm pointing toward you.
• 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.
• Typically only the vehicle detector closest to the intersection is designed to detect bicycles; sometimes only one in a bike lane detects bicycles, placing you where you could be right hooked.
• 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 will 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 has to be lower to prevent it from responding to a vehicle in another lane. But it is most sensitive over the wires.
• A quadrupole loop, a better kind, — has a wire down the middle — one of few instances of special infrastructure for cyclists which encourage lane control. A quadrupole loop usually works for bicyclists who place their wheels on its centerline.



• A vehicle detector with diagonal wires inside a polygon or circle is sensitive over its entire area.

Making a vehicle detector work

• An inductive loop vehicle detector responds to the metal of the wheel rims and/or metal bead wires in the tires. Except with detectors which have diagonal wire cuts, wait with wheels directly over the loop wires and in line with them. Wheels only an inch to the side are less likely to be detected. You may want to lift and reposition your rear wheel and zigzag your front wheel back and forth to get them over the wires.



• Skinny tires on larger wheels bring the rim closer to the ground, and are more likely to trigger the vehicle detector. (Motorcyclists may have worse problems than bicyclists!)
• The thickness, weight and magnetic properties of the rims and/or metal bead wires don’t matter. To the long-wavelength radio signals from the detector, they look like solid discs. 
• 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 so you have wheels over two sides of the loop. Make a right-hand “do not pass”‘ signal to establish lane control.



• With carbon-fiber wheels and Kevlar-bead tires, you might wrap a wire under the rim tape. You must connect the ends so it forms a loop.
• Happily, you need to trigger the signal only when no motor vehicle will trigger it — so getting into lane-control position is easy.
• Magnets that attach to the bicycle frame have been 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 the bicycle and aim your headlight upward at it.

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. You cross the intersection soon after the vehicle ahead of you triggers the signal.
• 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 hand right-handed “don’t pass” signal. Too bad! But other than that you aren’t 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 may work together to make this happen. I have succeeded with a group of cyclists who had no previous knowledge of vehicle detectors. I explained the strategy at a rest stop beforehand.
• No luck? You might push the pedestrian button, if there is one. This also can be useful to get a large group across a wide intersection. I thank Alan Muchnick for this observation. Scroll down for his comment giving details.

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.

Thanks to Steve Goodridge for some of the illustrations in this article. He has published an article with more technical detail about vehicle detectors, in case that might be of interest for you.