(no subject)

[jack]

You know those long black "wires" in the road that measure vehicles passing? It never occurred to me to wonder *how*. I think when I was about five I assumed they were wires, and it was all done with electricity, and then it never came up again and that assumption was buried somewhere in my mind.

(I've commented before on how you can acquire new common sense that gets applied to *new* facts you hear, but there's still a clutter of old facts where you remember you took them as true at the time, but not why, so can't separate out the really plausible from the spurious without re-examining them.)

The other day (when I went for a walk round the dug up railway line), I walked right by one, looked, and doh. It's pneumatic, isn't it? Or so I assume.

That's such a simple and elegant solution it seems inevitable. Have a tube, tied off at the far end. Have a sensor by the side of the road, that measures sudden pressure when the tube is momentarily constricted by a vehicle. Two tubes measure the speed.

On the other hand, I don't know if I'd have ever *thought* of that as a way of measuring it, if I'd had to solve the problem without ever seeing one.

ETA: http://groups.google.com/group/uk.d-i-y/browse_frm/thread/dadc7b3f4c78a5e/b87a895ebd15b76d?hl=en&lnk=st&q=traffic+monitoring+road&rnum=1#b87a895ebd15b76d describes them too, though I'm not sure if they have a more specific name than "pneumatic traffic counter"

Comments

[innocencest.livejournal.com]

Well, obviously measuring speed requires two readings of distance and time, and you have to have some way of detecting vehicles as they pass, so beyond that it's just a matter of method. I had always assumed they measured pressure (as opposed to, for example, bouncing light waves off them).

I think a more interesting challenge is how to distinguish two sufficiently close vehicles, or a vehicles with irregular numbers of wheels. However, if you're just trying to detect the presence of vehicles for traffic purposes, it's not really an issue.

[hmmm-tea.livejournal.com]

Not sure I'd have thought of it either.

I think the "paint lines on the road and point a camera at it" solution is more obvious. Far less elegant though.

[cartesiandaemon.livejournal.com]

I think a more interesting challenge is how to distinguish two sufficiently close vehicles, or a vehicles with irregular numbers of wheels.

LOL. Yes. If you go and stamp heavily on one wire, will it thereafter record a large number of extremely long concertina-like vehicles passing, all exactly five feet apart? :) It must be able to re-sync over any gap, or any pair of wheels travelling at significantly different speeds.

[stephdairy.livejournal.com]

If you mean the wires in the road that cause traffic lights to notice waiting vehicles, they're inductive loops.

(S)

[cartesiandaemon.livejournal.com]

I don't think so, but I don't know. I did know about that, but had forgotten it. Those are buried, right? These are just laid on top. And the one I saw was just tied off at the end, it didn't look to have a predetermined length (which would be inconsistent with having a circuit loop?).

[stephdairy.livejournal.com]

Inductive loops are buried, but the ones on the surface might very well be what the US DoT calls pneumatic road tubes which work exactly as you describe.

(S)

[d37373.livejournal.com]

Since you know the distance between the two sets, you can also do clever stuff to work out the wheelbase of a vehicle. Extreme example: a Smart car might hit the first tube with both sets of wheels before getting to the second tube. Lorries will hit both tubes with the front wheels before the second set of wheels get to the first tube.

Obviously it's a bit inaccurate, and screws up on a busy road with traffic in both directions, so the main statistic they use is a half-count for every thumb, and have 3-wheelbase-lorries count as 1.5 cars :) In that case, I believe the two sets are used to determine the direction.

You have to be very heavy to set one off with a person or a bike :)

[cartesiandaemon.livejournal.com]

Thank you, that sounds right. (A bit of a disappointment, a name so unoriginal I already guessed it, but there you go, thanks :))