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Gas mileage is greatly reduced if you have to suddenly stop because a traffic light is changing just as you get to it. Accordingly, I and most of us have gotten into the habit of modulating our speed so as not to get caught at lights. But what about the first light you get to after driving on a highway for a good part of your commute? You have no idea where it is in its cycle, so sometimes you just have to waste all that kinetic energy, right?

Wrong! I had a hunch that it might be possible to time this light by reference to a precise clock. Since all lights run on utility power, and many are computer controlled, you can count on the precise frequency of the power line and/or network synchronization of the computers to keep the lights on schedule. All you have to do is determine what the schedule is!

GPS time is accurate to the second (nanoseconds, actually, but they're not normally displayed). Every day for a couple of weeks I logged the green-to-red transitions of the light in question using a GPS clock. I then sorted the times and found a very simple pattern. The light would change every 1.5 minutes (exactly), with minutes divisible by 3, i.e. 3, 6, ... 57 having a change at :00 seconds, and intermediate minutes, i.e., 4, 7, etc., changing at the 30 second mark, 4:30, 7:30. This pattern repeats every hour, again exactly. The only exceptions are when there are no cars waiting on the cross street, in which case the light remains green in my direction for that cycle.

With this timing info and a GPS clock, all I have to do is make sure I don't arrive at the light at the wrong time. This is readily accomplished simply by slowing down if necessary a minute or so before getting to the light.

Yes, of course I know this is a bit silly. Yes, the GPS weighs something and uses a little energy, which reduces my gas mileage, although certainly not as much as my mandatory front license plate ((NJ) PRIUPS). And yet, one must keep his nerd credentials in order, and fanaticism in the pursuit of mileage is good entertainment and no vice.

Richard
 

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That works right up until you get to a place where the lights are controlled by sensors in the road.
 

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Well, no Melgish, Richard may have a point that I hadn't thought of.

I always thought lights in some areas are demand driven. That is, the light doesn't change unless someone is waiting for it to change. If someone approached the intersection on a cross-street, the light would change within one minute. However, if the light had just changed to let someone ahead of this car, then it would wait x minutes before allowing the change.

But having preset change intervals that are skipped if no cars are sensed makes a lot of sense.
In Hudson, there are a string of lights on US19 that change way too frequently, especially late evening. I find this especially true on the Little Road intersection. If the light just changed back to red for them, and green for me (me being on US19), they should wait at least 5 minutes.
But I suppose I wouldn't be saying that if I was the one waiting at the red light.
 
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