As Bob eluded - it's not a wire - it's a hose. One end of the hose is
capped and the sensor on the other end watches for a pressure change.
I've seen the dual-hose variety in at least two versions. in one the hoses
are different lengths. Both hoses cross one lane and only one hose crosses
the other. A wee bit'o math later and you can get individual values for
each lane.
The other version has both the same length. I don't have any real knowledge
of the internals but after watching one get installed... they spent a fair
amount of time controlling the distance between the hoses. I concluded
they're doing something interesting with the time between the sensors.
Easily enough should be direction (A before B or B before A) also
reasonably possible would be vehicle speed.
Either of those is subject to the occasional glitch when multiple vehicles
pass over at about the same time. Considering the typical purpose of such
measurements, I would expect even a 90% accuracy rate to be more than
adequate and I suspect these much better than that.

-Denny
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