The car would always have other sensors to fallback on, but this would reduce the amount of time that the DDT disengages due to inclement weather or other road hazards.
Add to this emitters in cones and possibly emergency vehicles.
I think useful "smart" road infrastructure would necessarily be horribly complicated, since the point of it would be to offload otherwise necessary complexity from the vehicles using the road. Perhaps the road could track the real time kinematics of all the cars on the the road using some combination of cameras, lidar, UWB, etc. It could then communicate that information via RF to the vehicles that want to receive it, along with local mapping information.
The automotive industry has been excited about "vehicle-to-vehicle" comms for the past 15 years aka V2V. Technically, all the standards that have emerged from the industry's efforts seem lame and nothing interesting has come of it. The FCC granted a chunk of 5Ghz spectrum for automotive DSRC use, and have subsequently taken back most of it for lack of use. It's still allocated for automotive, but it's now in the hands of the cell modem providers to do something useful with it.