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Old 12-04-2013, 01:18 PM
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Interesting ACC/FCW interaction

Yesterday's commute had a bit more challenging than usual, and ended up having the FCW go off 4 separate times. One mildly interesting thing is that all 4 times were with the ACC on. Also note that all of them were due to traffic slowing down in front of me (i.e., no cross-traffic, turns, etc.). Since the FCW is supposed to trigger if it thinks a crash is imminent without further actions, I'm a little surprised it would trigger with ACC on, especially since the ACC seemed to be braking appropriately for each of the situations. I suppose I could understand them wanting to warn you with these sudden changes so that you're prepared to take over and don't rely exclusively on the ACC. But I still find it interesting.

The more interesting one was when I decided to actually take over and touch the brake. In my opinion, the ACC was braking appropriately for the situation, but I decided to touch the brake anyway. As soon as I touched the brake, it started braking even harder than what the ACC was doing, more than I was doing, and more than I thought was necessary.

So for the most part the FCW was acting the way it's described. With the audio/visual notification going, if you start to brake and it feels the braking is insufficient, it will increase the braking. The thing I find curious though is that it seems that the FCW is more "conservative" than the ACC.

Now I do have the FCW set to Far and I have the ACC set to "3 bars" (e.g., 1 bar less than its most conservative setting). So there is potentially a slight difference in the conservative settings. But given that the ACC should maintain a safe following distance, and FCW is trying to mitigate a crash, you'd think the ACC braking would be more conservative.

It also seemed to me that the FCW was more conservative at lower speeds than at higher speeds (somewhat paradoxically). There were a couple times when I was traveling at about 70 and traffic slowed rather suddenly, but I let the ACC do its thing and it braked pretty darned hard (and caused me to have my foot hovering right over the brake because I was starting to get a little "pucker factor") but the FCW never went off. Yet in the other instances, where the FCW did trigger (still while ACC was on), I was traveling between 30-50mph and the ACC braking was not as hard (nor was the pucker factor as high) as the instances at higher speed. Very unscientific and very objective, I know - but that was my perception.

Anyway, still absolutely LOVING the ACC - I can travel almost my entire 50 mile-each-way commute without ever touching the gas or brake. And I'm still leaving the FCW set on far. Just found the behavior a little perplexing in some of the situations.
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Old 12-04-2013, 01:30 PM
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Re: Interesting ACC/FCW interaction

That is how the system works. Just wait until some car is turning slowly out of your lane and you can see it is no factor but the FCW doesn't see it that way and will apply the brakes. I love the interaction of those systems.
What you experienced is normal but takes some getting used to since many times our tolerance is much closer than the systems. Also given how jerks will cut in front of you to gain one car length I don't use it in traffic like that because it will cause braking all the time and I think in the manual it says something like not using it in those conditions.
Teriffic technology...
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Old 12-04-2013, 01:56 PM
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Re: Interesting ACC/FCW interaction

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That is how the system works. Just wait until some car is turning slowly out of your lane and you can see it is no factor but the FCW doesn't see it that way and will apply the brakes.
Well, as I said, the main thing I found interesting was that the FCW applied more brakes than the ACC did. i.e., the ACC seemed to be braking appropriately for the situation despite the fact that the FCW was flashing BRAKE BRAKE and then when I took over from the ACC by touching the brake, the FCW then applied even more brake than the ACC had been applying, and more than I would judge necessary.

I can fully understand the FCW being more conservative than *me* - I can judge the overall scene better than it can - I've also been through high performance driver training and have done my share of that. But the fact that the FCW was more conservative than the ACC is what I found most interesting.
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Old 12-05-2013, 12:54 AM
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Re: Interesting ACC/FCW interaction

So I think the ACC can only apply 40% of the braking force, and gives the BRAKE indicator when it detects that may not be enough braking. I don't know if the "additional braking" feature has such a limit.

The only FCW warning I've had that was strange was while traveling on an undivided two lane road at ~45mph with the other direction in bumper to bumper traffic. I suspect the FCW though I was about to drive into oncoming traffic.
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Old 12-06-2013, 01:00 AM
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Re: Interesting ACC/FCW interaction

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So I think the ACC can only apply 40% of the braking force, and gives the BRAKE indicator when it detects that may not be enough braking.
Interesting. I don't recall reading that about the 40% braking for the ACC, but I read the manual when I made the order back in August, so surely I've forgotten some things since then. Maybe it's time to re-read.

That certainly would explain what I experienced.

If the ACC can only do 40% braking, that's really important information to know as I shouldn't count on the ACC being able to properly brake in all situations. Of course I already don't rely on it completely - whenever traffic is slowing down quickly I have my foot hovering over the brake ready to take over if needed, but I *had* been assuming that it would always brake sufficiently. Now I'll probably be more likely to take over in some of those situations.
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Old 12-06-2013, 11:22 AM
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Re: Interesting ACC/FCW interaction

lol I guess its sorta off topic but does touch on the OP... The guys that constantly cut cars off out on the freeway switching lanes back and forth constantly trying to get ahead only to be 5 car lengths in front of you at the end of the traffic jam annoy me to no end.

Phew. Thanks, I just needed to say that.
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Old 12-07-2013, 07:13 PM
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Re: Interesting ACC/FCW interaction

An additional thought... per page 215 of the Owners manual.
"...If the driver reacts to the warnings by braking and the system determines
that the driver intends to avoid the collision by braking but has not applied sufficient brake force, the system will compensate and provide additional brake force as required."

In other more understandable words... most FCW w/crash mitigation systems precharge the brake system when it detects a possible collision event, those that don't auto brake will apply up to full brake pressure once the drive presses the break in reaction to the warning. I'm not sure if the the new system on the 2014's apply automatic full braking or some lesser amount under "emergency" braking conditions.

People can often be not expecting such a reaction (full brake pressure) and in some instances, such as slick icy conditions, can be a undesirable response. When driving in very Icy conditions, the driver should make a judgement call to leave the system active or turn it off and rely on focused attentive driving. As many know driving on ice sucks and many "automatic" features just aren't suited for it.
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