ACC Brake Warning

[DM]

Has anyone with ACC experienced this?

Whilst driving with ACC active at about 50mph I got an audible alert and 'depress brake' (or something similar) flashed on the multi-function display. I was not close to the car in front. The only thing that I can think is that I was joining a motorway in roadworks where the slip road was marked by cones. It's possible that a cone was a bit out of alignment and that the ACC sensor may have identified it as a stationary vehicle. The warning was only instantaneous and did not re-occur.

However my nataural reaction was to hit the brakes - fortunately it was only a light dab before my brain took over and pursuaded me that there was no reason to brake.

 

[Loz]

I get this from time to time, crisp packet, bird flying across the road..
It ignores stationary stuff like cones.

Sometimes you can be cruising along with noting in site and " bong " it drops out with, no sensor view. Caused by a build up of traffic film or dead moths etc, a quick wipe next time your stopped and all is well again.

 

[llackford]

I had ACC fitted on my old Seat Leon and this used to be the case with mine from time to time. But like Loz has mentioned a quick wipe down with a baby wipe and its all sorted. We have ACC on our new Cali also so will make sure i keep it clean

 

[Roger Boeken]

It also reacts when overtaking cyclists, indeed normally not to stationary objects as LOZ stated..

 

[Stoneybroke]

Very, very simply - ACC uses a radar; radars uses pulses of microwave energy and distances are calculated -further away, longer to bounce back, closer then shorter. Changes in these pulses relate to speed and distance. Cones, railings, spokes on bikes or motorbikes, wobbling curtain-sides on wagons, or roller shutter doors on vans, all cause the radar pulses to change and may fool the system into thinking the speed or distance is different to reality...
Radar guns that the police used to use, which were the same basic tech as ACC radar, were calibrated/verified with a stationary object travelling at (according to the radar readout) 50mph - the stationary object was a tuning fork! The vibrations of a particular note were interpreted as a set speed!
So ACC may interpret other reflections incorrectly.