In Forbes, Brad Templeton has a great article on the strengths and weaknesses of radar, relative to other sensors, and why so many companies seem to be interested in it right now. He pegs the analysis on Waymo’s recent radar announcement, but Templeton does a really nice job covering lots of different aspects of radar generally, including how Tesla and Mobileye do (and do not) use this sensor.
Because radar tells you how fast a target is moving towards or away from you, those targets stand out from all the stationary things in the world. You get reflections from stationary objects (like a stalled car in front of you) but it’s hard to tell reliably from all the other stationary things — like the road, the fences, the signs and more. Early radar users had to just ignore any returns from fixed objects, which is why you saw radar-equipped Teslas plow into the side of trucks crossing the road and emergency vehicles stopped in the left lane.