That's the untapped energy, is it not? If you don't trap it, it goes to waste.
The emerit physicists among us may be able to model this much better than poor engineers can.
But my take on this, at first sight, is that it's just going to increase air friction for passing car, so increase drag - I can't see how it could be "neutral". Would be like generating power from air flow on a car.
Now if you or someone else can come up with an elaborate model of passing cars, roads, and those sideway poles, and prove that it absolutely *doesn't* increase drag for passing cars whatsoever, have at it. I'm all ears. =)
Now, sure, it's probably going to be a tiny fraction of each car's energy - so you could consider it "negligible". But as negligible as the power you're going to harvest from them.
Point is, however tiny the fraction of energy harvested from each car passing by, it's still going to come from the cars - and thus, from the energy they themselves consume. Which means, the contraption would be just equivalent to taxing car owners with the corresponding amount of harvested energy, and use the same source the cars use (fossil fuels, electricity...)
And of course, beyond that, there's the cost of installation of maintenance, which is an additional indirect tax. So this just looks like yet another big green lie just to make an excuse for taxing people more, without making anything better in the end.
Now, again, if you can solidly demonstrate that the whole thing wouldn't create extra drag, I'm all ears. At this point, I do no buy that.