I've heard people on the internet claim that, in automotive radar the first thing they do when processing the signal is discard any stationary objects. Apparently this is because the vast majority of the time it's a sign or overhead gantry or guard rail - any of which could plausibly be very close to the lane of travel thousands of times per journey - and radar doesn't provide enough angular resolution to tell the difference.

Personally I've never seen these claims come from the mouth of an automotive radar expert, and many cars do use radar in their adaptive cruise control, so I present it as a rumour, not a fact :)