Some traffic lights won't change unless they sense an automobile or a pedestrian presses a button. This is a pain in my neck, because I'm a huge hippie and I cycle everywhere.
In many states, it is illegal to cycle on sidewalks. Generally this law is not enforced, but many cyclists, for reasons of comfort, expedience, and not shaking the hell out of their nice road bikes, prefer cycling in the street. This makes pressing traffic light buttons somewhat difficult.
Also, evidently, bikes do not trigger the sensors in the street. This probably makes some kind of sense, with physics or somesuch, but that won't buy me a cup of coffee. Or, you know, let me cross the street legally and with minimal risk to my life.
So, thanks for thinking ahead, traffic planners!