Sigh. Would you guys please go read the other threads where this is discussed ad nauseam? This challenge isn't unique to Tesla.
When using radar, it is very difficult to distinguish the difference between a stationary car in front of you at a distance and a fixed object that may be on the side or above/below the road. Imagine the road turns right up ahead, and on the left side is a fixed object. To the car, before it understands the road turns, that object could be a stopped car or something on the side of the road. Obviously we don't want the car needlessly slowing down or stopping for something on the side of the road, so AP tries to use the camera for more context. But the radar works further than the camera.
The way Tesla AP normally determines the object ahead is in fact a car is to track it while it's moving. We know the speed at which we're traveling. If the closing distance rate exactly matches our speed, it's possibly a fixed object. But if the closing distance rate is ANYTHING else, it's a moving object, and AP will tag it as such. Now if it stops, we still understand that it's a car, and we should slow down.
The net result is if a car in front of you is moving, even just a little - 1mph will do - when it comes into range of your radar, you're good. If the car in front of you is already stationary when it comes into range, AP must use other sensor data for context, and it may make a decision late or a poor one.
This works just fine for highway traffic as well as stop and go traffic during rush hour. Where you get into trouble is more rural areas with hills and turns and infrequent stops where it is more likely for you to come around a corner/hill at speed to stopped cars at a light.
