Trip the light fan-traffic

Updated
Inductive loop detector sweet spots
Inductive loop detector sweet spots
HumanTransport.org

Last Friday, Melissa mentioned that one of the new laws taking effect with the start of the new year is that bicycles and motorcycles in Illinois are allowed to run red lights after stopping for a reasonable amount of time.

The reason a law like this isn’t completely ridiculous might not be immediately obvious to everyone - particularly if you don’t ride a motorcycle or bicycle on the street - plus, there’s an interesting explanation that even those who have to deal with the problem often aren’t aware of.

Everyone who drives is already familiar with trip lights, right? You drive up to a red light light, usually on a side road with the kind of traffic volume that doesn’t justify being on a timer, and somehow the light becomes aware that you’re there and gives you a green. Sometimes you don’t get the green and you maybe roll back or forward a little before the light realizes you’re there.

Except often with a bike, the light never sees you and you just sit there like a mope. With no cars coming, logic dictates that after a little while (at one point the Illinois governor wanted to make it officially 120 seconds) you should just go. But with the proliferation of red light cams, you’re likely to end up with a ticket in the mail for doing so.

The actual new law specifically says it’s for when “the signal has failed to detect the arrival of the motorcycle due to the motorcycle’s size or weight,” but in fact, weight has nothing directly to do with it (nor size really, so much as conductivity).* The wire rectangles you see in the stopping zone in front of the light aren’t scales, and no amount of bouncing in your seat is going to help you change that light. Trust me, I’ve tried.

Instead, there’s electricity flowing through those wires, creating an electromagnetic field. When something massive and metal disrupts the field, that’s how the light knows you’re there. Sometimes your skinny bike isn’t enough to sufficiently disrupt the field - or else your relatively smaller size on a bike is allowing you to stop someplace where the field isn’t.

I’d been thinking of the electromagnetic field in visual terms, like those demonstrations with the iron filings on the sheet of paper on top of a magnetic bar, but this explanation makes the comparison to metal detectors, which makes a more useful metaphor. A trip light is like a giant metal detector and sometimes your bicycle or motorcycle is like the ring you forgot to take off at airport security but the alarm didn’t go off because it wasn’t enough metal to trigger detection.

The best explanation I found of what’s actually taking place is here from How Stuff Works. Those rectangles in the street contain coils of wire, making an inductive loop. That loop generates a field when electricity runs through it. The size of that field is determined by the number of loops and what the wire is made of, but it’s constant - it has a specific capacity. When you put a big piece of metal in the middle of it, like a car, you change the size of that field (I think you make it bigger actually). When the traffic light’s monitoring device detects that change,** burn rubber, baby.

They do sell devices to help amplify your bike’s presence, but as the diagram above from HumanTransport.org shows, there are also sweet spots in the road where you’re more likely to make your presence felt.

*As fun as this factoid is, it’s a terrible bar bet because there’s always someone who absolutely insists that they know a light somewhere that trips because of weight. I’m sure one exists somewhere, and I obviously haven’t checked every trip light in the world, but…

** The standard unit of measure of inductance of a coil is the henry. Heh.

OK, corrections? How’d I do?

Trip the light fan-traffic

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