[jfrej]

You are right, GPS alone doesn't. But it's augmented with both specialised maps (from the private companies you mentioned) and sensor data. Some self-driving cars use detailed 3D maps containing information about e.g. traffic light positions and have technology to read road signs and lights. Just like human drivers do.

I used to work for TomTom and at the time they were just getting into the self-driving cars sector. I don't know where they are with it now, but four years ago they were already building high-definition 3D maps for autonomous cars. And they had road sign reading technology.

You can see their HD 3D map in a video [1] where they also explain how it's constructed using their mapping vehicles.

EDIT: To make my point clear, we don't really need to invest in smart roads - the cars and the supporting tech are already smart enough.

[1] https://www.youtube.com/watch?v=ga5fW-QSXp0