By diverting the energy of each car to electrical production you are, in effect, levying a tax on automobiles. It would be cheaper and more efficient merely to pass a tax law.

The approaches to Chicago's many toll change baskets might be a place where the technique would make the most sense, but I'm not sure how long it would take to recapture the cost of the machines.

I think this idea has some merit if you only place them in areas where cars need to be slowing down anyway. But it might be difficult to approach at speed and trust that it will do your speed reduction for you--I'll probably still want to hit the brakes.

Also, how to do it without jostling the occupants is a question.