[Cyph0n]

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.

[TheKarateKid]

The current is even able to pass through the tires?

[Cyph0n]

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.

[Dreami]

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

[vageli]

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).

[BinaryIdiot]

Really interesting! Thanks for the correction!