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Discussion Starter · #1 ·
I'm putting this out there not as a complaint, maybe as a warning, but more so because I'm curious what I experienced.

I was on the freeway in sluggish traffic and I activated the Travel Assist.

I was in the left lane and we were probably traveling between 20 and 30. Traffic began to bunch up and slow and Travel Assist was doing a fine job of slowing progressively and generally pacing the deceleration of the vehicle I was trailing.

Suddenly the trailing cars slammed on their brakes. I saw it coming, but I think the two cars ahead were a little off their game, and maybe they panicked a little. I decided to let the Travel Assist work it out, since I felt like I had enough room to jump in if necessary.

Any how, the car immediately in front of me did that last minute "dive onto the shoulder" move -- I'm not sure if because he thought he was going to hit the car he was following, or as a preventative measure to keep me from hitting him.

The weirdest thing: as his car was about 3/4 over the edge line on the shoulder, but still partially in the lane, I guess the Travel Assist recognized a clear lane ahead. I'm not 100% sure exactly how it went down, but it either decreased braking application, or even began to accelerate (I couldn't tell -- it happened in the blink of an eye), and I quickly mashed the brake to keep any of this from continuing.

Obviously this isn't something I can easily try to replicate. Also, in a situation such as this driver intervention is 100% expected. I'm not trying to blame the system for anything, I was just sort of testing out its reaction. On that mark, it did really well, I felt like the ID.4 was completely at ease handling everything up to the lane diver. I'd love to see what it would have done in that last half second after I stomped on the brake, but of course doing so kills everything and I wouldn't let it get that far.
 

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I've had ACC on a Subaru Outback, a Crosstrek and my current Tiguan. They have all behaved as you describe when the cruise speed is set to faster than you've been going in traffic and the car immediately in front of you departs the lane. Your car will momentarily speed up until the sensors pick up the next car in front of you (Subaru uses an optical camera system and VW uses a radar system.)

It is one of those situations where a human driver knows what to expect, so doesn't accelerate unnecessarily while the ACC computer is just following its programming as the sensors see the objects immediately in front of them depart or come into range.
 

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I've never used any sort of adaptive cruise control before so I definitely wanted to read the manual. That section in particular stood out to me because it seems potentially dangerous. I hadn't even thought about your scenario, so thanks for posting!
 

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My first car with ACC was the Subaru Outback. I first used it with an attitude of complete distrust. That lasted several months until I had gained confidence that I know what it did well, and where and how it could be tripped up. ACC is a wonderful feature, but it does have its limitations. In particular, there have been some pretty spectacular accidents involving Tesla's autopilot system in which drivers got a bit too lazy for their own good.
 

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I'd better rrfm. I'd have thought with radar plus camera it would have been a different response. But, more importantly, thanks for confirming what I think I thought I observed. :)
Even Teslas react like this and there's no setting to adjust acceleration rate. You could always adjust the gap setting to be more conservative but that re-acceleration can be very abrupt and disconcerting when you're so far below the speed setpoint.
 
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Discussion Starter · #8 ·
I've had a good spread over the past 9 years, a Flex and S60 radar based, a crude camera-only in the i3, and now this magic. I am curious about how Subaru's stereo vision platforms in comparison.

My first cruise I installed from a kit from Radio Shack, in a 4-speed Datsun pick-up. It would switch off whenever I used the turn signals. I've come a long way, baby!
 

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I can't give you a back-to-back comparison of the Outback's camera system against the ID4 because I won't get the latter until sometime this summer. However, I do have ACC on my 2018 Tiguan and they are more similar than different. I do slightly prefer the Subaru system as it seems just a bit more natural and smooth about the way it handles things, but the differences are minor. I only used the ACC on the ID4 briefly but it seemed very similar to my Tiguan, albeit the lane centering is a feature my Tiguan doesn't have.
 

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Your note reminded me: I had an odd experience the other day. On a downtown one-way street (4 lanes - 1 parking on each side), speed limit of 30, I had turned on Travel Assist and it did well with the stop and go (signals at most intersections). A car in front of me changed lanes and there was perhaps 200+ feet open until a double-parked car stood, with its flashers on. I was intrigued that the dash didn't show the silhouette, so I allowed the ID.4 to continue (and slightly accelerate). As my car grew increasingly close to the parked car, it never appeared - blank screen. Finally, I hit the brakes, and the car ahead was never recognized. It seemed kind of worrisome!
 

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I don't think ACC is designed to recognize completely stopped vehicles. That has always seemed odd to me, but as far as I know most ACC systems are that way.
Well, that explains it! But how crazy is that? Moving or not, it's an object that my ID.4 is about to collide with :rolleyes:
 

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Discussion Starter · #13 ·
I don't think ACC is designed to recognize completely stopped vehicles. That has always seemed odd to me, but as far as I know most ACC systems are that way.
I understand this and didn't consider it for what I wrote in the original post, but it makes sense.

It's a choice the engineering team has to work out: if the system (whatever it's made up of: radar, camera, ultrasonics, lidar) detect a "pop-up" stopped object while traveling at highway speed, how reliable is the system, and what is the likelihood the system will slam on the brakes full panic stop and risk a rear-end collision? I think in this scenario the choice is very black-and-white, the driver needs to pay attention and take responsibility because false positives occur all the time. This scenario would be like the Tesla that broadsided the tractor trailer, where the driver was reportedly watching a movie.

In a normal driving situation, the system tracks a lead vehicle in motion, a positive lock of sorts, so if that vehicle decelerates rapidly the system is already aware of it and can safely stop brake as necessary (low probability of a false positive).

In my scenario, didn't expect the ID.4 to respond the way it did, but mainly because we were almost stopped when it reduced braking effort. I suppose I expected that between the radar and the camera, it would correctly recognize and not ignore the new threat 30 feet in front of me, which "looked" like a car (camera) and presumably "sounded" like a car (radar). But with the graphic that Leigh shared in Post #3, that's pretty close to an identical scenario.
 

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Hmm, that's interesting. It certainly explains why stopped cars at a light won't register to the system when someone leaves the lane. The problem is the brakes aren't robust enough to panic stop depending on how close you are.

I've been in two situations where I definitely felt I was on the knife's edge of the brakes when someone left my lane when I was only about two cars away from the (stopped) lead car. Imagine coming in to a stoplight, you're down to about 10 mph, someone abruptly leaves the lane, and your vehicle now jumps into a rapid acceleration back up to 35mph+ and you've only got about 20ft to react.

I don't know the solution but it's a jarring experience and I'm glad I was the one handling the wheel when it happened or we would have rear-ended the car stopped at the light.
 

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2021 VW ID.4 1st Edition, Dusk Blue <3
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Hmm, that's interesting. It certainly explains why stopped cars at a light won't register to the system when someone leaves the lane. The problem is the brakes aren't robust enough to panic stop depending on how close you are.

I've been in two situations where I definitely felt I was on the knife's edge of the brakes when someone left my lane when I was only about two cars away from the (stopped) lead car. Imagine coming in to a stoplight, you're down to about 10 mph, someone abruptly leaves the lane, and your vehicle now jumps into a rapid acceleration back up to 35mph+ and you've only got about 20ft to react.

I don't know the solution but it's a jarring experience and I'm glad I was the one handling the wheel when it happened or we would have rear-ended the car stopped at the light.
I've been familiar with this behavior for a few years, since my Alltrack behaved the same way. I pretty much just cancel the ACC if I notice the person in front of me starting to leave the lane while coming to a stop, and then re engage it once the car in front does get detected.
 
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