(no subject)
Oct. 9th, 2007 03:58 pmYou 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"
(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"