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Comment by fidz | Hacker News Reader

Such thing does exist? I thought traffict light only time-based on no sensor involved.

Depends on the area but yeah many intersections use sensors (cameras, sometimes under the road pressure sensors, etc) to make anywhere from subtle to extreme changes based on traffic patterns. The under the road pressure sensor has been around for decades.

When I was a kid there was one light that, when you drove over the pressure sensor, it wouldn't really do much. But if you backed up and drove over it again it must have registered an additional car coming through and the light would almost immediately go through its light cycle to change. It was really interesting to see!

Nowadays I think it's mostly cameras? We have a light near my home and the left signal will literally never trigger unless someone is in one of the left lanes.

It's not a pressure sensor, but an induction loop. Basically, there is a coil placed on the road that has a small AC current passed through it. When a car (metal) sits on top of the coil, the two "coils" couple, changing the overall inductance. A simple sensor can detect this change.

The current is even able to pass through the tires?

No, the current passes through the coil, and the coil has no physical contact with the vehicle.

Look up inductive coupling. The basic idea is that a changing (AC) current in a conductor generates a changing magnetic field (Ampere's law). This changing magnetic field then induces a voltage in the second conductor (Faraday's law). This is the principle behind how transformers work.

As far as I understand, it's just electromagnetic waves. No current passing through to the car, but the coil can "register" a change in its magnetic field and can determine that it's a car and how fast it goes

Can confirm. Also interesting to note that motorcycles often have trouble triggering these sensors (a common trick is to stick a heavy duty magnet underneath).

Really interesting! Thanks for the correction!

As a sibling comment noted, it's not a pressure sensor [1] but an induction loop.

The Wikipedia article https://en.wikipedia.org/wiki/Induction_loop#Vehicle_detecti... has fairly extensive details on modern implementation.

[1] These do exist for weight-in-motion systems, however.

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