Sub-pavement signal loops are actually just large magnetic sensors. Weight doesn't factor into the equation. You just need a large (enough) mass of ferrous metal to trip the loop sensor. Some are set more sensitive than others, for reasons unknown to me. My father was a traffic signal technician for the city of Leavenworth for like 7 years, so my information comes straight from the source. Seek the source!
Also, I have noticed that they're starting to put motion sensors on top of the signals instead of loops in the pavement, at least around here. It's been great, I can actually go places without waiting for two years for a green light.