Cars don't use magnets, but the sensors are sensitive to metal mass. If you use a magnet make sure to align it directly over the black cut out lines in the pavement. I don't use a magnet, when I pull up to a traffic signal I make sure either my front wheel or rear wheel is stopped directly on the thin black lines of the sensor. I understand the wire is the sensor and have never had a problem with this technique. I have also heard if you are stuck at a light that will not cycle, put your bike in neutral and drop your kickstand onto the line and that will trip the light. Don't forget your kickstand down or you'll stall as soon as you engage your shifter.
