[Symmetry]

They have a fixed position radar which sees the distances and radial speeds of objects in a cone extending in front of the car. Because it can't tell you which direction an object is in they have to filter out objects that are radially stationary so as not to be triggered by things on the side of the road, overhead signs, etc. At least until these objects are close enough that the cone's width doesn't exceed the width of the car by much. Now, it's very useful when dealing with other moving cars which probably aren't hanging in the air above your direction of travel. But it doesn't help with everything.