Are you sure it's a weight sensor? It's usually an inductive loop[1], which detects metal. They're usually supposed to detect motorcycles, but are sometimes incorrectly calibrated. I've been able to trigger most troublesome detectors by tilting my motorcycle while on top of it.

Some people also put magnets in their boots[2].

1. https://en.wikipedia.org/wiki/Induction_loop

2. http://www.instructables.com/id/Traffic-Light-Trigger-for-yo...