Hi-tech sleeping policemen that use the pressure of passing cars to create power

If the plates are used in places where you want to slow down traffic anyways (eg replacing speed bumps), then it's a good idea. But for other places, it might actually be a wasting more energy than just using electricity normally.

 

I saw this whole idea proposed and discussed somewhere else a few years ago. In a way it is a direct (localized) tax on those that use that particular road. I have to imagine that it would create some extra noise as the cars drive over the generator.

 

Yes, what they said.
This is a much better idea: LED-encrusted speed bump flattens out when you're not flat-out - Engadget

 

Maybe only place them on downslopes.