That’s a fair point, a device could theoretically harvest energy that would have otherwise been wasted, and that would be green energy. I imagine a wind system could work, though it might result in cars experiencing additional drag from slower wind speeds.
However, the piezoelectric generators mentioned in the article quite clearly do not use waste energy. They compress under the weight of the cars, turning a small amount of gravitational potential energy into electricity. That energy must be made up with extra fuel.
Finally, even if all of the vehicles on the road were powered by clean electricity, it would still be a useless system. Piezoelectrics are nowhere near 100% efficient, so you’re just taking electricity from the vehicles at a loss.
I mean, I can think of many ideal places for such things. Intersections for instance, where cars will be wanting to slow down or come to a stop regardless. Speed limit changes, where you want cars to slow down. Even in place of speed bumps, assuming you can get enough of a drag on the car to effect a slow down.
