Hacking cars

Nope, the range would be much too short. (And still uses RF, specifically RFID.)

Keyless fobs use RF.

Chris

Transmitting all the time?

I think they are passive devices - they respond to RF power transmitted by the card machine or ticket barrier, either by selectively absorbing part of the spectrum, or using that power to generate a response transmission.

Clearly the potential for a jamming signal to disable the function.

It's make/model dependent.

Some have back-up RFID for when the fob battery dies. Earlier VW (and maybe others) use the perimeter alarm to create the unlock zone; this was a notorious (car) battery killer, even though it disabled after two weeks or so.

The systems are sensitive enough to know whether the fob is inside or outside the car, and even which door you are near.

Chris