Can you disable ACC?

[ColinB]

I think I read somewhere that   although heavy braking by the collision avoidance systems does activate brake lights  braking for ACC speed moderation  only applies them if retardation is  severe enough to need them.Which is not normally the case.  I might be wrong.  If necessary try to  pre-empt the systems likelihood of braking.

Isn’t that the same as any other car? If you simply lift your right foot and allow engine braking to slow you down, then there are no brake lights (although an advanced driver may touch the brake pedal lightly in order to warn any following drivers). So surely ACC is just mimicking that? Or are you saying that ACC applies the brakes - and the brake lights - every time it decides it’s getting too close to the car in front?

[Kremmen]

From my experience, the unexpected reduction in speed was certainly more than engine braking, especially as to use ACC you need to be in 'D'

[Lord Voltermore]

You expect it to brake when traffic ahead suddenly slows significantly . But yes there are times when it brakes slightly  when an alert human  would have anticipated the need to slow earlier and eased off the throttle without needing to brake.
Its not severe  and its not always  but,as Kremmen says, its more than just engine braking. It  can feel  a bit strange at first having something else braking for you when you dont expect it. I've not had it happen often enough to analyse when it might happen, or its possible affect on following vehicles.

Its partly due to the fact a human can react proactively by reading the road well ahead.  The  the aid can only react to what its  sensors 'see'.   

This also shows  when a  car suddenly enters your lane close in front.   An alert driver may spot its about to happen , due to driver behaviour ,indicators etc   reacting  even before the car has left its own lane.  The 'system only spots it once  the car is well into your lane . A driver with good hazard perception may notice a perceptible delay between the need to react and when the system reacts .Its only a fraction of a second delay but enough that a human might react a couple of metres sooner in distance covered than the ACC ,which then needs to compensate by braking that much harder. . Its better than nothing. Basic CC wouldn't react at all.  (At  70 mph a car travels 31 metres per second. ,or 4 metres per second if there is a speed differential of 10 mph. That's a cars length.) 

 I'm not criticising it as such.  I'm not sure how with current technology it could be improved. It cant think ahead like a human.  Maybe a faster 'refresh ' rate on the sensor.   But it does  illustrate the need for the driver to remain alert and in control, and only use it with discretion depending on traffic and weather conditions.

[Neil Ives]

When I had my scare while using ACC I wished that it had a narrower field of vision;  the system sensed a slow moving vehicle that was in the lane to my left. Later, it detected a lorry that was parked in a layby. Both times the brakes were applied hard.

[Kremmen]

I had similar and I found that if I was plumb square in my lane then it worked as intended, albeit further back than I expected, but when changing lanes it seemed to read other lane traffic so as you say maybe the beam is too wide.

[Lord Voltermore]

When I had my scare while using ACC I wished that it had a narrower field of vision;  the system sensed a slow moving vehicle that was in the lane to my left. Later, it detected a lorry that was parked in a layby. Both times the brakes were applied hard.

That sounds more like the kind of thing the   Collison mitigation braking system CMBS  might do.  The fact you had ACC on may just be a coincidence.
   On fast A roads  you may quite legitimately pass parked trucks quite close, and even on motorways you may be approaching the vehicle ahead quite rapidly knowing you have the overtake  safely planned.   But this could 'spook' the CMBS which may 'assume' a collision is possible.  I've only had this happen once so far with a stopped bus.  I approached at a speed consistent with knowing I could pull out to overtake once past a traffic Island . The CMBS panicked a bit.
You can reduce the sensitivity of the CMBS between Far normal and near. (page 155 on line manual) Or even switch it off.

ACC may be getting unfair flack.