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2018 Dark Blue Pearl Outback 3.6R Premier
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Discussion Starter · #1 ·
When using adaptive cruise control it seems it is unable to detect a vehicle ahead if its stopped (the manual does say it may have difficultly with vehicles at a significantly different speed or stopped). A couple days ago I was following other vehicles on a two lane highway using ACC. The vehicle in front of me slowed down to turn right off the highway. ACC slowed me down accordingly. As he turned right, the vehicles ahead of him came to stop on the highway due to construction. As soon as my ACC lost sight of the vehicle that turned right, it did not detect the next vehicle down road that had stopped, and it accelerated hard towards the stopped vehicle. It was unnerving, pre-collision braking would have kicked-in but of course I braked before that happened. I don't understand the technology enough to understand why ACC would have trouble with a stopped vehicle ahead when obviously pre-collision braking system does not.
 

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2015 Legacy 3.6R ES, 2014 Forester Touring ES, 2005 Jeep Liberty Limited
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When using adaptive cruise control it seems it is unable to detect a vehicle ahead if its stopped (the manual does say it may have difficultly with vehicles at a significantly different speed or stopped). A couple days ago I was following other vehicles on a two lane highway using ACC. The vehicle in front of me slowed down to turn right off the highway. ACC slowed me down accordingly. As he turned right, the vehicles ahead of him came to stop on the highway due to construction. As soon as my ACC lost sight of the vehicle that turned right, it did not detect the next vehicle down road that had stopped, and it accelerated hard towards the stopped vehicle. It was unnerving, pre-collision braking would have kicked-in but of course I braked before that happened. I don't understand the technology enough to understand why ACC would have trouble with a stopped vehicle ahead when obviously pre-collision braking system does not.
It is made as a driver assist not a self driving function. It is best use is (ACC) when on open roads or highways not best suited for high traffic areas even though some owners engaged it for stop and go.

Laughing at Oneself and with Others is Good for the Soul
 

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2016 Tungsten Outback 2.5l Premium w/ES, OP 14, PP #4
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I'm sure you've observed there's a detection range. When ACC "locks" on to the controlling vehicle in front there's a chirp and the indicator turns green. The range is a function of how many bars you've selected. I use 3 most of the time but 4 would extend the detection range. Not knowing how far ahead the stopped car was, maybe ACC hadn't locked onto the stopped vehicle yet. I understand it's disconcerting, you as the driver, sees something stopped but your car is accelerating.
 

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2018 3.6R Touring
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I'm sure you've observed there's a detection range. When ACC "locks" on to the controlling vehicle in front there's a chirp and the indicator turns green. The range is a function of how many bars you've selected. I use 3 most of the time but 4 would extend the detection range. Not knowing how far ahead the stopped car was, maybe ACC hadn't locked onto the stopped vehicle yet. I understand it's disconcerting, you as the driver, sees something stopped but your car is accelerating.
I don't think that's how it works. I think the detection range is whatever it is (and no doubt varies some depending on circumstances), and setting the different bars just tells it how close or far you want to stay from the leading vehicle.
 

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2018 3.6R Limited
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If the ACC is tracking a vehicle, it will indeed take you to a stop if that's what the tracked vehicle does. That it would accelerate you into a stopped vehicle immediately after losing the tracked vehicle, however, could suggest the system's acquisition time is too long relative to your circumstance. For example, if you're traveling 15m/s, the stopped car is 40m away, and ACC takes 3 seconds to acquire a target, you're going to collide (absent braking) before the stopped car is tracked ([15m/s x 3s] > 40m). There's no way to know for certain without seeing the programming logic, but it seems a timing limitation.
 

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2018 Dark Blue Pearl Outback 3.6R Premier
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Discussion Starter · #8 ·
If the ACC is tracking a vehicle, it will indeed take you to a stop if that's what the tracked vehicle does. That it would accelerate you into a stopped vehicle immediately after losing the tracked vehicle, however, could suggest the system's acquisition time is too long relative to your circumstance. For example, if you're traveling 15m/s, the stopped car is 40m away, and ACC takes 3 seconds to acquire a target, you're going to collide (absent braking) before the stopped car is tracked ([15m/s x 3s] > 40m). There's no way to know for certain without seeing the programming logic, but it seems a timing limitation.
That certainly sounds plausible - good thinking! But I'm thinking collision avoidance would not take that long, different analysis/algorithm I suppose.
 

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2018 Dark Blue Pearl Outback 3.6R Premier
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195 Posts
Discussion Starter · #9 ·
I'm sure you've observed there's a detection range. When ACC "locks" on to the controlling vehicle in front there's a chirp and the indicator turns green. The range is a function of how many bars you've selected. I use 3 most of the time but 4 would extend the detection range. Not knowing how far ahead the stopped car was, maybe ACC hadn't locked onto the stopped vehicle yet. I understand it's disconcerting, you as the driver, sees something stopped but your car is accelerating.
Yes, I was in the detection range. I do keep the range on maximum distance. It was fine, I easily braked in time. I just did not initially understand what happened and thought the system malfunctioned. I went back to the operator's manual and it does state that is may not detect stopped vehicles or vehicles travelling at a significantly different speed.
 
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