The energy is being expended anyway. In this case, you are actually making each car that passes over the bump more efficient and reducing carbon by translating the kenetic energy developed into a source of energy for another use. We also have to keep in mind that the property owners pay taxes to build roads that the motorists use, so if I rolled one of these bumps out in front of my house, each one of the 500 commuters coming into and out of my neighborhood would be "paying" me back for my investment in our local roads. Seems to be a fair trade off.
I can see how these devises could generate power, but I wonder if the cost in fuel for the vehicle to drive over them would be more expensive than the power they generate. It could well result in costing 100 dollars to generate 10 cents of power.
Before they start putting these things all over, I hope the spend some time to calculate the true cost involved. Plus, if the speed bump damages a vehicle, who is liable?
It gets interesting when you start looking at all of the implications.