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Comment by dalyons | Hacker News Reader

And yet I have it in my very real world (and unexciting) Subaru for years and it never does what you say. I’m guessing it has some idea of differential speeds , not just distance-to-object

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I too have a Subaru, my emergency brake has only engaged when trying to parallel park a bit too aggressively. I must be a very good driver! But perhaps it has saved my bumper a few dents

so if someone merges over into your lane where you are it doesn't activate? I must not understand the point of this feature then.

On my Honda if a car merges in front of me and is accelerating, it maintains its speed presumably on the assumption that the separation will continue to increase. If a car merges in front and is slower than me, i.e. separation is decreasing, it will brake in an attempt to maintain the set following distance.

In practice it does leave a bit more following distance than most human drivers, but your worry of constantly getting cut off until you come to a stop in the road is not a real thing. This is partly because the car modifies the set following distance based on speed, so that the same setting is much closer at e.g. 5 mph than at 60.

Also, at least on mine, it doesn't attempt to implement the full follow distance right away. Mine can tell a vehicle is accelerating away from me and won't slow down to fit the spacing on the expectation that the spacing will soon be achieved by the other vehicle. If the vehicle in front of me is going slower than my set speed and not accelerating, then it slowly will lag back to get the set follow distance. You do have to worry about aggressive cars that cut you off in heavy traffic as the vehicle will just let them and lag back, but I can at least adjust the follow distance to partially mitigate this situation.

OK, it's good to know that it won't stop. But if it slows automatically it will still result in the same thing. All the other drivers just continue to cut you off indefinitely.

Actually, maybe I misunderstood you. I thought you were talking about adaptive cruise but you may have been talking about automatic emergency braking.

Interpreting your original comment in that context: AEB only kicks in at a point where maximum braking will result in you barely not hitting something in front of you, or maybe even hitting it at a low speed. Modern cars can stop very fast indeed with maximum braking so in practice this threshold is never crossed under normal driving conditions unless the software returns a false positive. It's not about maintaining a safe following distance—it's about stopping the car at the extreme edge of the safety envelope, beyond which no manual or automatic intervention could prevent a violent crash.

False positives do happen but I think they are very rare or practically nonexistent in good implementations. I've never had it happen with my Honda.

Theoretically OP's concern is still possible with AEB but it would require non-trivial coordination between offensive drivers where each subsequent driver comes in at a lower speed than the previous, cuts OP's car off -- now slowed -- with the same distance as previous in order to force a standstill. It would be a neat stunt driving trick.

Someone else explained, but AEB is not cruise control. It tries to stop you if and only if it genuinely thinks you are about to crash into a stationary object or and object traveling at such a different speed that it might as well be stationary. Ie, a potential crash, not an everyday lane merge situation

It’s for when your mother in law hurtles your family towards a truck in front of you with no indication of trying to slow down (personal experience). The car does its best to stop acceleration while maximizing braking force safely.

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