ACC and Brake Lights

[SimonB]

I have searched the forum and not found a specific answer about whether ACC puts on the brake lights when it operates and slows the Cali down.
Normally it would not be much of an issue, but on Friday we were heading down the M25 and a car came up on the offside and going quite a bit faster than us. He then put his hazard lights on and shot diagonally in front of us from the outside lane braking hard and heading for the hard shoulder.
I had set a 50 foot gap between us and the car in front so there was plenty of room for his manoeuvre so I didn't brake. The ACC didn't have all the information I had and, I assume, saw the obstruction in front of us about 25 feet ahead and slowing so it applied the brakes very firmly, but safely.
It was not a stamp on the brakes, lock the wheels job but a firm slowing starting from gradual to quick. As soon as the other car was out of the way it let the brakes off again.
My concern was whether the vehicle behind me who probably didn't have ACC would have seen brake lights to warn them that we were rapidly slowing down.
Conversely when cars come down a slip road and, rather than slowing down, accelerate to go between you and the car in front to get into the 3rd lane, the ACC doesn't react as I assume it sees the vehicle as accelerating away rather than getting nearer.

 

[Loz]

Yes it operates the lights when braking but not when engine braking.

 

[Duncan]

It will only operate the brake lights when it uses the brakes, if it is changing down gear to slow you down the brake lights will not come on.

 

[CampervanTime]

I had ACC on my last Audi S3 and yes, it puts the brake lights on when required

 

[SimonB]

I had ACC on my last Audi S3 and yes, it puts the brake lights on when required

Many thanks - I thought it should, but having experienced a fast decrease in speed I thought it wise to check.

 

[CampervanTime]

Yes it’s quite some technology and it can hit the brakes pretty hard if it wants to. Quite reassuring really.

 

[kurienp]

As others have mentioned - yes it does.

The behaviour of the ACC is quite interesting and knowing it better helps you determine when it is right to intervene. it seems that the ACC follows a half-car rule.
a) If a car is merging from a slip road, I know that ACC will not sense the presence of the car until half of the car is in front of me, until then, I am carefully watching the car's manoeuvres to ensure that the car in front does not have any obstruction to slow it down. Once my ACC has sensed the other car, then I am relaxed that ACC will intervene if need be.
b) if the car is front is slowing down to either change lanes to take a right or a left turn, the ACC will also slow down my car. If I feel that the car in front has no obstructions to complete the manoeuvre, to ensure that the flow of traffic is maintained, I will gently nudge the accelarator so that ACC does not slow down my car and the vehicles behind continue to flow.

As you very rightly say - if the car ahead is accelarating, ACC does intervene ever so slightly, but depending on the distance to the car in front, you can feel the brakes pre-load. That helps for a faster stop. Has definitely helped me once.

There is one turn in my town where the two lanes come quite close to each other, narrowing before it diverges ... and that has fooled my ACC many times. When there are turns - I DO NOT TRUST the ACC. My fingers are on the speed controls actively reducing the speed to maintain distance from the vehicle in front (no feet on the brakes still ). Yes, I use ACC in town also and that is one of the best tech features IMHO.

A rule of thumb to keep ACC working in a turn is - the top center of the steering wheel should be pointing toward the vehicle in front Even then, do not trust the ACC in a sharp-ish turn.

 

[Stoneybroke]

I always though it would be useful to have a dash light saying when the ACC was braking to confirm the brake lights are on...
Agree with the observations of @kurienp - though cars can go wholly into a deceleration lane and the ACC sometimes thinks they're still in the way.

 

[Teejay]

As others have mentioned - yes it does.

The behaviour of the ACC is quite interesting and knowing it better helps you determine when it is right to intervene. it seems that the ACC follows a half-car rule.
a) If a car is merging from a slip road, I know that ACC will not sense the presence of the car until half of the car is in front of me, until then, I am carefully watching the car's manoeuvres to ensure that the car in front does not have any obstruction to slow it down. Once my ACC has sensed the other car, then I am relaxed that ACC will intervene if need be.
b) if the car is front is slowing down to either change lanes to take a right or a left turn, the ACC will also slow down my car. If I feel that the car in front has no obstructions to complete the manoeuvre, to ensure that the flow of traffic is maintained, I will gently nudge the accelarator so that ACC does not slow down my car and the vehicles behind continue to flow.

As you very rightly say - if the car ahead is accelarating, ACC does intervene ever so slightly, but depending on the distance to the car in front, you can feel the brakes pre-load. That helps for a faster stop. Has definitely helped me once.

There is one turn in my town where the two lanes come quite close to each other, narrowing before it diverges ... and that has fooled my ACC many times. When there are turns - I DO NOT TRUST the ACC. My fingers are on the speed controls actively reducing the speed to maintain distance from the vehicle in front (no feet on the brakes still ). Yes, I use ACC in town also and that is one of the best tech features IMHO.

A rule of thumb to keep ACC working in a turn is - the top center of the steering wheel should be pointing toward the vehicle in front Even then, do not trust the ACC in a sharp-ish turn.

This is exactly how I use mine. Always keep the ACC only 5-10mph above the actual limit. Also have mine set to economy so it doesn't launch me after the obstruction has cleared..

..me..