My understanding the systems work not through true magenetism but the fact that when a metal object is placed in a coil of wire it will change the voltage. yes this is some to do with magnetism but the voltage change from a metal object does it. The engine in the car is the easy and obvious piece that changes this. A motorcycle also has fairly significant amounts of metal.

Unfortunatly bikes are made of very little amounts of metal and we are not carrying much either. Those carbon fiber bikes truly don't stand a chance because it will not affect the voltage.

With this information I guess they could make it super sensitive and notice the smaller changes but then it will pick up cross traffic or other signals and screw with the lights and make everyone get annoyed.

Go press the crosswalk button if you need, that works if they are available.