My understanding is that your typical automotive radar will have insufficient angular resolution to reliably distinguish, say, an overpass from a semi blocking the road, or a pedestrian standing in the middle of the road from one on the footpath.

Radar does however have the advantage of measuring object speed directly via the doppler effect, so you can filter out all stationary objects reliably, then assume that all moving objects are on the road in front of you and need to be reacted/responded to.

So I think it's the case that radar can detect stationary objects easily, but cannot determine their position enough to be useful, hence in practice stationary objects are ignored.

Adaptive cruise control is solving a totally different problem. It is specifically looking for moving objects to match pace with. That's very different from autonomous driving systems.

Radar is quite good at finding stationary metal objects, particularly. Putting it in a car, if anything, helps, because the station objects are more likely to be moving relative to the car...

The newer “4D imaging” radars can track stationary objects too. This is what the self driving companies have turned to in recent times.