I have a vision for the future of driving. Right now, every road in the united states, except for highways in rural Montana, has a speed limit of some kind. This limit is communicated with bright white signs with black letters, explaining the speed in miles per hour. (The US is really stubborn about metric.) Drivers are expected to obey the signs. Where there is no sign, the limit is 25MPH (~40KPH) near any business or residential area.
Now generally the drivers can see the signs, so this isn't a problem. This is a deterrent against automated driving, though, because visual recognition is extremely poor with computers. They literally cannot tell what they are looking at.
So I think we should embed a wire in all lanes of the road, and the section of wire will emit a weak signal, which the car can pick up, and the signal shall tell the car how fast it may go. The car should indicate this in the speedometer, which will make it instantly clear if one is speeding.
This has the advantage of, if conditions suddenly change, the speed limit can quickly change too. Decrease all speed limits by 20MPH instantly when fog rolls in.
This will be more helpful, though, for automatic driving cars. When the car can detect nearby obstacles, and also has a good idea of what the speed limit is, it can confidently move forward. I don't want the car going 70MPH, just because there are no immediate obstacles.