[Blackbeagle1]

Can someone help with this? My wife is the sole driver of our SXP. She takes the same road home every day. She has noticed a weird thing everyday. I can't explain it.

So she's on a narrow highway...55mph speed limit. Her cruise is at 60-65 (following the flow of traffic). Then as it opens up the speed limit goes to 70mph. But for some reason the car slows down at the EXACT spot every time. The car is going from 70mph, and it slows down to 55mph...at the exact same spot. I can't figure it out. I have guessed it has something to do with the Navigation Cruise where it detects the change in speed limit. But slowing down doesn't make sense because the speed limit actually went up from 45mph to 55mph to 70mph. She read somewhere where the car sees a shadow of bridge and thinks it's another car and slows down. I find that hard to believe.

Can anyone explain this phenomenon?

 

[UN-Limited]

That's a new one. So to be sure I understand, she has CC enabled and set to 70mph in an area where 70mph is the posted speed limit? When it suddenly drops to 55, does the dash display the posted speed as 55 or 70?

 

[Leshemdoc]

Can someone help with this? My wife is the sole driver of our SXP. She takes the same road home every day. She has noticed a weird thing everyday. I can't explain it.

So she's on a narrow highway...55mph speed limit. Her cruise is at 60-65 (following the flow of traffic). Then as it opens up the speed limit goes to 70mph. But for some reason the car slows down at the EXACT spot every time. The car is going from 70mph, and it slows down to 55mph...at the exact same spot. I can't figure it out. I have guessed it has something to do with the Navigation Cruise where it detects the change in speed limit. But slowing down doesn't make sense because the speed limit actually went up from 45mph to 55mph to 70mph. She read somewhere where the car sees a shadow of bridge and thinks it's another car and slows down. I find that hard to believe.

Can anyone explain this phenomenon?

I probably have questions more than answers:
If the speed limit is 55 and she sets the CC to 60-65 she is not activating/setting the car to change the speed according to speed limit, instead she is having the car only signal that the speed limit is higher or lower than she is driving. Only when you set the CC to the speed limit and the speed turns green then the car will change the speed to the speed limit. I learned this here on the forum and someone else probably explained it much better than I am. Bottom line seems like her car is not set to change the speed based on limits from what you are describing.

In that case there are 2 other reasons I can think of why the car would slow down on its own- one is a car ahead - might be a coincidence and probably an unlikely explanation that there is always a car there in the same spot causing your car to slow down.
The other is navigation based cruise control. My understanding, but have never tried because I only use Waze, is that the smart CC is set to slow down based on information from the navigation system if there is a curve on the road or you are reaching an exit... Again I read something to that extent - but have no idea if and how it works, but based on what I remember - it is another reason why the car will slow down if it is not set to follow the speed limit.
Hope it helps...

Does your car reflect the speed correctly as being 70 when it changes to 70 or does it show 55?

 

[bmn78]

Is it a speed limit 55mph in the area, according to GPS map in the car? The car probably can change it's speed accordingly, but you can change it in the setting. Try this, and please let us know if works:

 

[Blackbeagle1]

I've seen this. It's my first thought that this is what's happening. But it still doesn't make sense. So it's a long highway stretch. It starts at 45 (and nobody goes 45), then changes to 55, then after the bridge, it changes to 70. My wife probably has the CC at 65 the whole time. And it goes along just fine. But right at the bridge, where 55 will change to 70, the speed drops. It's almost dangerous because the car slows down for no reason. If there's a car behind her doing 65-70 and all the sudden she brakes, it could potentially cause a rear end.

 

[bmn78]

 

[bmn78]

I would try to deactivate "Highway Auto Speed Change" and see if helps.

 

[UN-Limited]

I've seen this. It's my first thought that this is what's happening. But it still doesn't make sense. So it's a long highway stretch. It starts at 45 (and nobody goes 45), then changes to 55, then after the bridge, it changes to 70. My wife probably has the CC at 65 the whole time. And it goes along just fine. But right at the bridge, where 55 will change to 70, the speed drops. It's almost dangerous because the car slows down for no reason. If there's a car behind her doing 65-70 and all the sudden she brakes, it could potentially cause a rear end.

Makes me think the car is registering a 55 mph speed limit and you've got your CC set to not exceed X mph over the speed limit. I've been on the freeway where the limit is 65 mph, but the dash says 45 mph because the GPS thinks I'm still on the feeder road.

 

[Blackbeagle1]

So in reading the Navigation based cruise control, how it works is like this: You set the CC to the speed limit. Let's say it's 55mph. You set it to 55mph. Then as the speed goes down to 45, it will also slow you to 45. Or if it detects you approaching a curve, it will slow down, then speed back up again. That's what i'm assuming.

But she's in a 55mph zone. The speed is set to 65-70. In less than a half mile, it will change to 70mph. WHy would it slow down? It slows down right at the exact spot each time.

 

[UN-Limited]

So in reading the Navigation based cruise control, how it works is like this: You set the CC to the speed limit. Let's say it's 55mph. You set it to 55mph. Then as the speed goes down to 45, it will also slow you to 45. Or if it detects you approaching a curve, it will slow down, then speed back up again. That's what i'm assuming.

But she's in a 55mph zone. The speed is set to 65-70. In less than a half mile, it will change to 70mph. WHy would it slow down? It slows down right at the exact spot each time.

You have to be sure the car knows the speed limit is 70. Check the dash, there will be a speed limit image and if it says 55 when it should really say 70, then you know the issue.

 

[JAFFO]

I have seen, driving on the 401 here in Ontario (Canada), some areas where the indicated speed by the vehicle is 100 km/h (matches the roadway) but as you pass an on-ramp or service center on-ramp the vehicle's indicated speed will show 40 km/h. Because most of the time I drive at 10 km/h over the limit, the van will not try to adjust the speed. I have never tried it at the speed limit to see what will happen.

It is almost as if the navigation is picking up the speed limit of the on-ramp and assuming that you are travelling on that stretch of roadway. I have heard of the navigation confusing destination addresses with buildings down the street or on the wrong side. Perhaps the navigation is shifted off and confuses the vehicle as being on the on-ramp and not on the highway? This could be dangerous if you are moving along with traffic and then the vehicle tries to slow down some 60 km/h to match its new found speed limit. You could get a transport right in your rear hatch!