I work in autonomous vehicle R&D, and if we had an environment filled with reliable sensors it would considerably simplify our problem.

There is no such thing as a _complete, detailed 3D map of driveable areas_. Most current efforts rely heavily on high resolution, large scale, semantically labeled maps, but on any given stretch of road this is only a first approximation of the environment. Live sensors embedded in infrastructure that could pass along real-time information and updates, particularly from directions our on-board sensors can't capture, would be very useful indeed.

And yes, road markings are part of the semantics we use.