Radar on the Teslas, if it works like how I am familiar with, just detects "things". Things that have a decent radar return. This class of things does indeed include cars and other vehicles, but should also include walls and such.

It is likely that they're using at least 2-D radar, so the system should be able to discriminate between an object directly in the path of travel, and one that is off to the side.

Sorry for not having quoted the exact paragraph in my very first comment. You can find the reference just above; it stops to work at around 90 mph (150 km/h) - Tesla Model S manual, Collision Avoidance Assist, page 95.

Additional limitations are listed in page 96.