Ehh it's still a rubbish idea, that money would be much better spent going after primary producers of energy, like solar, wind, geothermal, or nuclear.

Some napkin math and an equivalent area of solar, say over a road or parking lot would produce 3.5 million kwh in a year.

What kind of junk energy is there to harvest from a car (in meaningful amounts)? I guess breaking is the obvious answer, but that's already covered by regenerative breaking. Most car-based energy harvesting systems seem to employ speedbumps that clearly take useful (kinetic) energy away from the car (probably at a very poor efficiency).

IIRC there was a real world test of this idea some time ago and the results weren’t great. But imagine you do this in a tunnel- that will help improve the results and answer your question.

If you’ve ever been in a subway you’ve probably noticed the train pushes a wall of air ahead of itself. This is the energy you would need to capture. But by “capturing” it, you would inherently increase resistance on the airflow, and therefore the vehicle.

The reason you can’t imagine this being an issue in a typical roadside is the same reason the effect yields poor results. There is little in the way of channeling the air across the turbines, which also means there is little resistance on the passing vehicles. Of course the air in the atmosphere, pushing against the air your car moves ahead of itself, is the classic example of this overall effect.

How would a turbine that takes energy from the air current generated by a passing car decrease the energy of the car?

Not sure where you got that idea from, but how would that generate a meaningful amount of energy? It seems very unlikely that such a system would ever recover the energy spent on its construction.