Hi
The lamp post or sign post will cause drag also. The difference is that the turbine is designed to capture that air and create drag, and generate useful power, the lamp post does not capture as much air, has less drag, and just lets that power dissipate. With good design you can get that turbine to create more drag as a car goes past than a smooth lamp post. In the same way a row of trees will create more drag to a car than row of smooth lamp posts. The energy extracted from the car can be seen as the movement of the leaves and branches.
Do you want the cars to have as friction-less travel as possible (and use less energy) or extract some energy from them, turn it into power at low efficiency and then send that power to somewhere else and claim that that saves power?
Just look at the energy balance; if the turbines generate a significant enough amount of energy to be useful then that has to come at the expense of taking tiny amounts of power from many cars. It's not an efficient process.
Mike

