This one's come up before and there's some question as to whether it would increase the drag on cars if you actually used this 'wind.'

Also, putting the vanes on the shoulder would be claustrophobic and scary because they'll be way more effective if closer to the car. The vanes really need to be in the roadway--you can never crash into them and the distance factor is nil.

But again, I'd need to see testing to prove that we weren't just stealing the energy from the cars' gas tanks.

The real problem with this idea is that the amount of wind generated by cars is miniscule compared to the wind generated by nature. Why bother?

Here's a thought.

Shrink the vanes or even make them into small, horizontally-mounted road-side turbines at select intervals along the road. Use them to power local streetlights or traffic lights via rechargable batteries.

That way you won't feel claustrophobic, the drag is minimized on vehicles and you still generate power.

You may even shelter the miniature (2-4 inch) turbines from debris with a metal grating or screen.