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Old 01-24-2009, 01:27 PM
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djasonpenney djasonpenney is offline
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Join Date: Feb 2006
Location: near Nike in Beaverton
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Default Triggering Wire Loops...

Let me start by acknowledging that wire loops are a real PITA. It's especially annoying in Washington County, where the engineers are more interested in avoiding false signals (thus slowing down cross traffic) than they are in allowing legitimate road users to safely share the road; they routinely set the sensitivity to be markedly less than the nationally recommended standard.

That being said, I have found that 95% of these wire loops can be triggered, if you understand their principles of operation and apply those principles properly.

First of all, the wire loops use induction, not magnetism, in order to sense your presence. Induction is like when you put your hand on a cheap AM radio and the reception improves. Anything that conducts electricity, including titanium, magnesium, aluminum, or salt water (which is what you are mostly made of) have inductive properties. (These little magnets you're supposed to buy and put on your bottom bracket are pure snake oil!) The traffic light control recognizes when inductance on the sensor exceeds some cumulative threshold.

Second, the sensors are designed to not change the light if the inductance threshold is triggered only momentarily. Thus, the steps that one takes to exceed the inductive threshold must be maintained until the light changes. Stay put! Don't try to trigger the light and then move forward or to the right afterwards.

Third, these wire loops are most sensitive near the wire itself. The sensor triggers when the total inductance exceeds some threshold that is specifically tuned by the field engineer.

So, here's what I do. First of all, suppose it's a diamond shaped sensor; you've got four quadrants (upper left, lower left, upper right, and lower right). In this case, I stop over the sensor with my entire front wheel exactly over and parallel to the upper left quadrant. The rear wheel intersects the low left quadrant, and my left foot crosses both the upper left and lower left quadrants at the corner where I'm standing. I then hold the position until the light turns.

One could also mirror this right-side with a right-footed stop. However, I usually feel safer on the left side since I'm further out in the intersection and controlling the lane.

The round sensors are much harder to trigger. I know, it's not supposed to be that way, but there it is . I angle my front wheel to the right (like before), positioned so that it touches the pavement slightly inside of the circle and crosses the circle both in front of and behind the contact point. In geometric terms, I want the wheel to trace a "chord" that passes as close to the wire as possible. My rear wheel I try to position so the point of contact is directly on the wire loop. I then place my left foot directly on the wire loop at my left.

Like I said, this almost always works. There's only one complication that you should be aware of: rain. If the pavement over the wire loop is very wet (like in a heavy rain), the wire loop's sensitivity is further reduced. When you think you've mastered the wire loops on your route, a day will come when you have to trigger the light while standing in a real toad strangler. Just be forewarned: just when you realize you're getting really wet and cold, you'll just have to take your life into your hands and run the light. (Case in point is eastbound Butner Road at Cedar Hills Boulevard. Don't go there, Dorothy; it works just fine if it's dry or just damp....)

Finally, at least here in Washington County, the authorities are generally receptive to adjusting lights that won't change for bicyclists. They just don't tune them correctly to begin with. Just last week I saw a traffic engineer adjusting the left turn lane from northbound Murray Road onto Cornell Road. He sat at the control box and waved a full size pickup back and forth as he calibrated the sensor. "You pinhead!" I wanted to scream at him, "you should have a 100 pound middle schooler on a 30 pound mountain bike out there instead of a pickup truck!"

But now I'm just getting grumpy. Seriously, you'll find that most wire loops are perfectly tractable if you treat them property. Be safe!
__________________
ORS 811.065 (1)(a):

The driver of a motor vehicle may only pass a person operating a bicycle by driving to the left of the bicycle at a safe distance and returning to the lane of travel once the motor vehicle is safely clear of the overtaken bicycle. For the purposes of this paragraph, a “safe distance” means a distance that is sufficient to prevent contact with the person operating the bicycle if the person were to fall into the driver’s lane of traffic....

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