I wonder how much power it would generate if you had a gazillion of these things lining our major highways. I bet there's nearly a constant 10mph breeze from all those cars.
If your goal is to get free electricity by leaching off of cars, without regard to the environment, then it might work. But, there is a huge fixed cost to manufacture, transport, install, and test the machinery, and a non-negligible variable cost in maintaining the system.
Imagine the car is running in a vacuum and gets 30MPG. Now, add normal air resistance. It will not get 30MPG anymore. Now, add something else that further increases wind resistance. The car will get even fewer miles per gallon.