perpetual motion at Sainsbury's

This article caught my eye in the Guardian's technology section: a branch of Sainsbury's supermarket is installing speed bumps that harvest energy "for free" from the cars passing over them.

Hang on a minute. You can only get energy from passing vehicles if you slow them down as they pass. I guess this could validly produce energy that would otherwise be wasted, strictly provided the speed bumps are installed on a stretch of road where most drivers would brake (this is not pointed out in the article). Although for me, in my Toyota Prius, when you brake the car is supposed to recycle that energy by using it to recharge the battery, so if I pass over those speed bumps I lose out -- ending up powering the supermarket and not the car battery.