It all depends
@hoodedswan: It's not necessarily true that only vibrations that were there before are converted to energy. It depends on the construction of the sensors, which will change the elasticity of the roadway. If they don't match the elasticity that was there before, they could actually take more out of cars passing than was lost before.
Think of it as walking on a trampoline. The stiffer the trampoline the less work it takes to walk. If the trampoline is very elastic, it's as though you are always walking uphill. Just as you can vary the tightness of the trampoline to change the effect, so could the roadway sensors be made to take more energy out of passing cars than they were losing before to heat.
