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tl-w.gif (842 bytes)tr-w.gif (841 bytes)Frank 'N Fred

In which we trip the light fantastic.

Whoa! Whoa! Whoa! Hey, Frank, what do you think you're doin'? I saw you run that red light!

Oh, hi Fred. Boy, that's irritating.

You think you're irritated 'cause I caught you? Just wait till I tell everyone! Hoo, boy!

No, Fred. It's irritating that the light doesn't work right! That's why I had to go through the red.

What do you mean it doesn't work right? Hey, it's lit, ain't it?

Sorry, Fred. To be working right, a light has to change from red to green in a reasonable amount of time. I sat there for several minutes, just to see what would happen, then I treated it like a stop sign. I'm not required to spend all day at a red light!

Yeah, but look there! It just turned green for that car. See? It works!

And that's especially irritating. The inductive loop is set to detect cars. Its sensitivity isn't high enough to detect a bike. Blast, I'm going to have to make another phone call to get it adjusted.

Um... did you say deductive loop?

No, inductive. There's an oscillator circuit connected to an inductive loop in the road, and it's resonant frequency depends on the circuit's inductance. A car will change the circuit's inductance enough to be detected, but they've got it set too low to detect a bike.

Um... are you still speaking English? I don't even see a loop!

Oh, sorry Fred. I was speaking Engineering. Here, let's take a close look at the road. See that thin rectangular slot they cut into the road, then re-filled? It's about five feet wide and eight feet long.

Oh yeah. Did they bury something under that rectangle?

No, they buried some wire in the rectangle. They loop a few turns of wire around in that slot, then top up the slot with a filler. The wire goes to the control box over there. It uses an oscillating field in the wire loop to detect a car waiting for the light. After a reasonable wait, it changes the light to green. It doesn't waste time stopping the main road unless a car's waiting.

Cool! Oh ... unless a car's waiting! Now I get it. So it couldn't tell your bike was there, eh?

Right. I did everything I could, and it still wouldn't trip.

Hah! Did you pretend you were a car? Did you gain weight, or what?

No, Fred. Here's the trick: you look for the cuts in the road. When you find the rectangle, you ride right on top of the right-side cut - or the left-side cut if you're getting ready for a left turn. The sides are most sensitive to a bike.

If that doesn't trip the light, you can try laying your bike down sideways inside of the rectangle. That way your frame and wheels catch more of the oscillating field. But I tried both of those tricks, and the light still wouldn't trip.

Boy, they should put a button on a post by the curb, huh? Or hey, you could just go over and push that pedestrian button!

No good, Fred. A button at the curb isn't safe. Cars or bikes would run into it in no time. Besides, you can't expect a left-turning cyclist to ride over to the right, push a button, then waddle back to the proper left-turn position. That puts him in conflict with other vehicles. No, they've got to turn up the sensitivity, even if it causes false trips. Or they can put the proper type of loop in the road.

Wait, what's that false trip thing?

With a simple rectangular loop, the oscillating field is shaped like a fountain. It comes up inside the rectangle, then sprays out and down around the outside. If they set the control for a bike inside the rectangle, the outside part sometimes responds to cars in the wrong lane. Traffic people don't like that.

So this can't be fixed?

It can! They can do a "figure eight" loop, for example, by sawing one more cut through the center of the rectangle. It costs a tiny bit more to put in, but it works better. The field then goes up one side and down the other. It doesn't spray into the next lane, and it's easy to set it for a bike. It's in the Department of Transportation manuals, but lots of traffic engineers just don't think about bikes. That's really irritating!

Wow, just one more cut and the lights work for bikes! It sounds easy!

Well, there's one more thing. They should paint something on the road to show a cyclist where to stand to trip the light. I recently learned that with some figure-eight rectangles, you can't be on top of the outside lines! The detectors couldn't tell I was there - but when I was dead center in the loop, they worked fine!

Holy cow. So you gotta be an expert just to get a traffic light to change?

I'd say, if you see a plain rectangle, be sure to ride over the side cut. If you see a rectangle with an extra cut in the center, ride over the center cut. Maybe we can get the traffic engineers to paint the proper symbol on the road, to show us where to ride. After all, the symbol's in the Department of Transportation manual, too. Then we'll all be Tripping the Light Fantastic!

- Frank Krygowski

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By Frank Krygowski











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