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Traffic Sensors

  1. Jun 11, 2003 #1


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    I'm sure you all have seen them, I just wonder if anyone knows what they do:

    I have noticed traffic sensors of some sort in various places over the last few months. They consist of two long wires which look like co-ax cable duct-taped across the road/street/lane/highway. They are connected to a box which is left on the side of the road (usually padlocked to something). The box is powered by a small array of solar cells on top.

    Anybody know what they do?

    I first thought they might be checking speed, but then they started showing up on highways. My hunch is that at highway speeds and volumes (and lane crossings, etc.), a device would need more computing power than could be provided by solar cells to decipher wheel passings and convert to average speeds.

    They may be checking volume - counting up the # of axles which pass by the sensor. But then why would it need two cables, and why have I seen them sometimes only going into 1 lane on the highway?

    I'm just curious if I should be speeding over them to mess up the data collection or not.
  2. jcsd
  3. Jun 11, 2003 #2


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    I've seen similar devices around here, but they consist of only one wire.

    I always understood they were traffic counters.

    Besides, the only feasible way to use it to measure speed would be having 2 wires so that there was atleast a foot or 2 distance, so it could time how long it took from wire a to wire b.

    I don't think something separated by inches would be accurate at all for a car tire.

    My advice for dealing with one:

    Equipment needed:
    Powerfull rearwheel drive car
    Knowledge of powerbraking.

    It can get nasty though, sometimes the cable gets tangled around your wheel, drag the box down the hwy.

    J/k of course.
  4. Jun 16, 2003 #3
    lol on the powerbraking.

    Yes, they are traffic counters. Probably just another way to justify needing more money for roadwork in many cases, IMO.
  5. Jun 16, 2003 #4
    Huh why not, I would have thought just the opposite. Depends on whether you record when the tire has left the wire or the when the tire arrived on the wire!
  6. Jun 16, 2003 #5
    I think they just count number of "bumps" or essentially the number of axles, in the ones I have seen anyways.
  7. Jun 16, 2003 #6
    Do you think the wires/tubes are filled with liquid ?
  8. Jun 17, 2003 #7


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    They can be either counters or speed sensors. I'm pretty sure in some states the box has a camera in it. You only need one wire for counting.

    Too slow. Many if not most are piezoelectric. I did my senior design project for THIS company which makes piezoelectric sensors for devices such as traffic sensors.
  9. Jun 25, 2003 #8
    In my area, the traffic sensors are in the gound and are engaged by a weight load, which I am fine with when I am driving...but it is a pain in the a** when your riding your bike :)
  10. Jun 29, 2003 #9
    Oh, so that's how the left turn light near where I live works. I've been wondering how it knows when a car is in the turn lane. There are no obvious cables and such.
  11. Jan 22, 2004 #10

    These "traffic counters" are just that. They are used for a wide variety of things and the state keeps records of these in an department actually called "Traffic Counts". Being in commercial real estate, I use these quite often to show to prospects so they can see what kind of volume their business will be exposed to on a daily basis. I hope this sheds some light.
  12. Jan 22, 2004 #11
    I don't think something separated by inches would be accurate at all for a car tire.
    Actually, the wires might just measure the time by pairs of wheels that go over it, i.e., how much time there is between the front and rear wheels. I don't know too much about these things, but it makes sorta sense, and the second wire might just be there in case there's a bump that makes the car hop a few centimeters and the car doesn't hit the first wire.

    If you have no idea what I'm talking about its okay. I don't either.
  13. Jan 27, 2004 #12
    Love that! Can I borrow your statement for other posts? Just kidding. A great line, though!
  14. Jan 27, 2004 #13
    They've been around for a while. As kids (30 years ago) my friend and I would gleefully jump up and down on them.
  15. Jan 30, 2004 #14
    You could do that now for money, upping the traffic counts. melarent and others might be interested
  16. Jan 31, 2004 #15


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    The counters I've seen work from sensing a pules of air pressure form tires runing over a rubber tube(or some flexable material, not a wire or cable).
  17. Feb 7, 2004 #16


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    I was having those same memories! (only it was 40+ years ago!)

    I have since wondered if all that jumping had anything to do with the street widening (it cost us one of our Cherry trees!) and paving that followed a few years later!
  18. Feb 20, 2004 #17


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    I worked in a Highway Authority some years back so heres the low down.

    Firstly, yes all they do is count traffic. They are normally two wires for either ensuring that the right direction of the traffic is being counted, or purely for redundency in case one gets damaged by a vehicle, as they are left out for sometimes weeks.

    The data collected is mormally stored at the Local Highway Authority and is used for a variety of things. Scheme design, future improvements etc. I more detailed example would be for PLanning issues i.e. if someone was going to build a new superstore then the road network serving it would probably have count data. This is then used as a base, the new store traffic added, which then allows the Highway Authority to ensure that any say traffic signalled junctions are upgraded to take account of the extra traffic from the superstore.

    Hope this helps
  19. Feb 22, 2004 #18
    The authorities wouldnt be able to attain reliable results this way as just about all models of vehicles have a different tyres sizes and gaps between axls, a large SUV could go over alot faster than a small car.
  20. Feb 23, 2004 #19
    I own a company that specializes in traffic counting services. So I will try and answer a few questions.

    Yes, those tubes that you see laying across the road and attached to a box are ATR's (Automatic Traffic Recorders). Two tube setups are the norm, not for redundency as someone stated, they are set-up with two tubes so that you can catch Two-Directional counts.

    The tubes are filled with air not liquid, the machine has air switches that register when a pulse of air is forced through the tube by a tire hit.

    These machines are far more advanced than many give them credit for, they not only register volume but also can do Speed, Classification, Gaps, Etc. If you were to go through one of my machines at specific time I could tell you not only how fast you were going but how many cars were in front of you as well as behind you. I would also be able to narrow down what kind of vehicle you were driving from a bycicle to an 18-wheeler, SUV or Sportcar ETC. By the way they have a 99.5% accuracy rate.

    The newer machines don't have any tubes at all, in fact you wouldn't even know they were there. They are about 5"x5"x1/2" mini-computers that send off a magnetic signal above them. As soon as a vehicle goes over it and breaks the magnetic field it registers.

    Hope this answered your questions.
  21. Feb 23, 2004 #20
    Neat physics - next they'll measure one's Blood Alcohol Content!
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