jcross1231

Hope this isn't a repeat of a previous post. If it is, I couldn't find it.
The adaptive cruise control on my '21 RDX erroneously slows down at times without apparent cause. It's happened on a clear highway. More often, it happens when I'm slowly overtaking a car in an adjacent lane when it's at my 10 o'clock or 2 o'clock position. It'll typically slow down 3-5 mph, then slowly figure things out and resume the set speed. Thing is, it takes a while for this "figuring out" thing to happen - maybe 20 seconds. On a 1,800 mile drive to Florida last month, this happened every time.
This isn't unique to my RDX. The wife's '19 Civic does it. A friend with a Tesla has the same issue. I suspect others do, also. Any idea what the cause is?

Riddix21

I beleive the sensors activates on a certain distance(10/2oclock). It is part of their safety features which is kinda telling us that you are too close. It happens to me all the time so now I change lanes before I come closer to the car that I want to pass also when a car infront of me is trying to turn left or right on a side street and Im kinda trying to squeeze to go straight. A bit annyoing but I got use to it.

jcross1231

I probably wasn't clear that this is happening when the lane I'm in is clear and there's a car in an adjacent lane.

Ron T

My '20 RDX does this on occasion. Whenever it happens it appears that the RDX thinks the car you are overtaking is drifting into your lane and is keeping an appropriate distance. This has been my observation most of the time, but there are times when it just seems it does it for no reason. When this does occur and I determine there is no reason to brake or slow, I accelerate manually to get past the vehicle instead of waiting for the RDX to figure it out.

Ron

4th Acura

I agree. My 2020 RDX behaves the same way. I’ve kind of gotten use to the occasional hesitation when passing in an unobstructed left lane. This behaviour is rare.

leomio2.0

I hate it. I don't use it anymore. I just use the regular cruise control. I've had something similar happen ... cruising in the right lane in a different state and a car exited the highway. Lane was now clear ahead and, for no reason, the car just laid onto the brakes. The person behind me almost rear-ended me and pulled alongside me cursing me out (I'm assuming ... by the angry scowl and rapid mouth movements).

Ironically, the only time I was thankful to have it was bumper-to-bumper NYC traffic. But I don't commute to NYC anymore, so I simply don't use it.

mvl

I've found a couple things with ACC. If an adjacent car moves even one inch into your lane, it is "in your lane" and ACC will slow down and not pass it.

But most of the time, especially around new curves or hills or poorly painted lane areas, the radar "hears" a car approaching by the distance ping decreasing, but the camera software hasn't recognized and confirmed it is in an adjacent lane yet, so the ACC slows down for safety until it the camera computer can confirm that the car is not in your lane. Happens more often on my 2020 MDX which has a lower quality camera and/or recognition computer than my 22 RDX.

I practice I don't mind, decades ago a car once pulled out of a line of slow cars next to me without looking and got into my lane, I had to slam my brakes like crazy to avoid hitting it. I've since learned never to pass a car more than 5-10 mph faster than it, to allow me more time to stop if it stupidly gets in my lane. This ACC's "bug" causes me to slow down when passing so I don't mind it.

jcross1231

Good points. Curves are an issue. The camera can't tell it's there, so a car can look like it's in your lane until the road straightens out. My '21 seems to be better at this than my '16 was. The "bug" is a pain when you're only overtaking another vehicle by 2-3 mph, since it slows down more than that. My '16 had a problem slamming on the brakes when passing a semi that was on the right when there was a fence-type barrier in the left median. All I can figure is the barrier created a modulation that made the range to the semi appear to be closing in a lot faster than it was. And yeah, it's a good idea not to overtake too quickly. There's always somebody who doesn't look before they pull out. I always wonder at those guys who go whizzing past you doing 30 mph over the speed limit.

subiefanIL83

I have also encountered the occasional phantom slowdown when using ACC. Not my first car with ACC, but the first one that had the phantom slowdowns. Its also not as good at holding speeds going down grades as my previous car.

ELIN

I've experienced the same on my '18 MDX Tech and '21 TLX A-Spec. When I was shopping for an X3, it didn't bother me that the Driving Assistance Professional Package that included ACC was not available due to the chip shortage. There was a chance that the BMW implementation could have been better but the prior Acura ACC history certainly soured on me getting it. ACC is just not "natural" enough for me to warrant the extra cost. You're more likely to get "electronic whiplash" in stop and go traffic!

Some folks used the lack of ACC as a demerit against purchasing cars like the cheaper BMWs but I think these same folks are just checking boxes and haven't really tried using ACC as intended.

mvl

When I'm not in a huge hurry to get somewhere, I just set ACC to 3 or 4 following distance and let cars cut in. With the larger following space it has more react time and has minimal whiplash.

With ACC and LKAS, I find it takes me twice as many hours to get tired and need a break during a drive then without these features. So I would never buy another car without the features.

jcross1231

Those are my thoughts also, and it applies to rental cars when I have a choice, too. I'm very happy with the ACC other than the phantom rollbacks, which have become predictable enough that I can compensate for them with the throttle.

ELIN

Follow distance doesn't matter if the person passing you on the lefty suddenly cuts into your lane! That 3-4 follow distance suddenly becomes 1 or less than 1! There's really no planning around this!

mvl

Agree that occurs if they cut in and slow down. But if they cut in and maintain speed the ACC gently slows until it regains proper distance. But if people do not cut in the 3-4 gives enough time for the ACC to adjust speed to react to normal traffic while being less jerky.

ELIN

I know we don't have a monopoly on drivers who live in the left lane but it sure feels like it!

jcross1231

Addikt

Glad to find this thread! I recently took a 3h drive and encountered this several times. I observed that it consistently happened with gray/silver cars and trucks. Anyone else notice this? Any theories on why it would only behave this way with this color vehicle? Any TSB out for this - it's very annoying?

ELIN

That is an interesting theory that it might be color related. However, we do need to rule out the base rate. Does there happen to be more gray/silver cars in that area?

Jmar55

I just put over 2000 miles on my new Aspec Advanced, which was mostly interstate with ACC on. Thankfully, never experienced this issue. FWIW, I averaged around 26 mpg burning premium at speeds between 65 to 80 with heavy air conditioning use.

mvl

I'm in an Integra loaner right now with the fully digital ACC display, and the display clearly shows the root cause of this this bug with phantom slowdowns for adjacent cars during road bends.

As the road gets further away (eg: 8+ car lengths / or about 3 cars in mild traffic with typical tailgating), ACC is unable to decipher lane divider paintings, and assumes the road continues to bend around the same amount that it is bending at closer distances. You can see this error as the Integra displays live bends that it calculates in the road, and you can see the mismatch vs reality further down the road all the time. However the cars are identified in their actual position as the radar can easily see them, and trucks are even easier for the radar to see very far away. (the Integra draws each identified car on the MID in its interpreted relative position, and is very accurate). This mismatch of cars placed actually where they are, with lane bends miscalculated further away, results in situations where faraway cars in adjacent lanes are misinterpreted as partially creeping into your lane. When this happens, if the car is much slower than you, the ACC starts slowing down to not hit the car. Once the car gets closer to a distance where the lane bend is interpreted properly, the car shows as not in your lane, and the ACC speeds up again.

Regarding the grey/silver, I think that color is more likely to confuse the lane line interpreter, as I also see that misdrawn lane lines do seem to occur more frequently near cars of those colors. Likely because they are similar colors to the lane lines themselves.

Texasrdx21

It's called "Phantom braking" - the system thinks it sees an object and applies the brakes. It happens rarely, but easily overcome with just pushing the "go pedal" which will over-ride the system. For example, driving the freeway and your in the left lane, if the vehicle in the right lane moves over to it's left line - the camera system may catch that object as coming in your lane (even when it is not) and apply the brakes.

rmeastma

I am just gonna change the name here to Rob and the model year to '19 and steal this post.

jcross1231

I finally figured out the cause of the slowdown - at least in my case. It's the shadow of the car in the adjacent lane. When it's casting a shadow that takes up more than about half of your lane, the ACC will slow down about 3-4 mph. I don't know what the system is thinking. If it thought there was a car in your lane, it would trail farther back as if it was following the car. It only slows down a bit, thinks things over and then proceeds. So who knows.

Addikt

🤔 i'll have to pay attention to that and see if i observe the same. Thank you for circling back on this!