What do the pressure sensors sense to make the light change, if there's no car around?
They're loop detectors (basically metal detectors), not pressure sensors, and they make the light change because they're damaged and the signal controller is always sensing the presence of a vehicle there.
What really makes me wonder is that most of the time, the agency responsible for the traffic light doesn't fix this problem until someone complains about it. It's like they either have no way of remotely monitoring the signal controller or don't know how to remotely monitor it. Or if they do they have no idea what to look at to determine if there's a problem (Hint: If the controller reports that the loop detector has been detecting a vehicle there continuously for days on end, IT'S PROBABLY BROKEN!)