Radars do just fine detecting stationary objects, what they can't do is tell whether the stationary object is blocking the road, overhead, or to the side of a c

Radars do just fine detecting stationary objects, what they can't do is tell whether the stationary object is blocking the road, overhead, or to the side of a curve in the road. As most radar reports of stationary objects fall in the latter two categories, the adaptive cruise control system filters them out of consideration.