Adaptive Cruise - Presense question

[Itguy]

Hi everyone

Does anyone know what can trigger pre-sense warnings with ACC?

I thought it was all due to distance but going to work the other day it triggered when a very slow moving car pulled into my lane quite far in the distance (75m ish). I was travelling at about 50mph and they were probably doing 20mph but accelerating.

This triggered my pre-sense warning but it didn't brake for me as I was just about to override the ACC brake anyway.

 

[Cockney Boy]

What car have you got?

 

[Itguy]

As per sig - new S3

 

[snowfree52]

It's the difference in velocity that triggers the presense.

 

[Itguy]

Yes, that certainly seems to be what happened to me. I don't know why but I always thought it was distance related?

 

[snowfree52]

well it's related. presense sees you are still accelerating that means you didn't see the car, and it starts to brake when it thinks it's the last chance before the accident

 

[RossR]

Like the adaptive cruise it's looking at three things - distance, difference in speed and difference in rate of acceleration (or deceleration).

 

[cemerson]

If it suddenly sees a car that it didn't see before, I think it's cautious for a bit until it can work out that it isn't a threat. If the car wasn't accelerating then it was probably just about to slow down. I've not seen the pre-sense warning come up in mine though for similar situations.