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There is more information you'd have to communicate. Like how long I can leave my car in this spot before I have to move it.

It could be conveyed: "no parking until 2PM" or "parking until 5PM" or "parking for 15 minutes."

We'll have self driving cars before intelligent city street signs, then just let the cars figure out where they can park.

And then it becomes "No parking until 2pm unless you have a green permit, in which case you can park until 5pm. Except if that green permit has a blue diamond in the corner, which means you can't park here for more than three hours".

None of the signs I see on the webpage have those conditions, though.

YES

Attaching a small solar panel to the top of the pole would take care of the energy problem. You can draw power from the grid at night (when there's spare capacity), or use a battery (a bit more costly).

e-ink?