Thread: Autonomous braking

Some time ago I fitted ACC to my MY12 L322 and enabled autonomous braking at the same time. I have forward alert enabled mostly in case it saves me from a roo strike at night, but it can be switched off using the service menu if it becomes too annoying in traffic.

Today Forward Alert was activated while cruising at 100 kph as I approached a vehicle that had mostly pulled off the road just ahead as the road curved slightly to the right, presumably because my steering was still straight ahead on the straight road just before the start of the bend and therefore aiming for the stopped car. However shortly after the FA sounded, my RR began an emergency stop and judging by the severity of the braking, it may have stopped before reaching the stopped car. The braking was aborted when I floored the accelerator thereby cancelling ACC but I doubt that the vehicle travelling too closely behind me could have stopped in time. That vehicle subsequently left a decent gap so the driver must have had a fright.

I had initially thought that FA had initiated the braking, which it should not, instead only pre-loading the pads against the discs in readiness for an emergency stop. However the pre-loading would have made the autonomous braking of ACC much more aggressive, so between them not a bad reaction.

Note to self: Cancel ACC or press the accelerator slightly to suspend ACC if in the same situation again.

My conclusion is that the braking was correct. My other experiences with Forward Alert have been in traffic when I've elected to steer around a vehicle pulling to the left to stop or turn, where I just ignore the alarm and keep to my intended path. I haven't encountered any documentation regarding upper speed limits for autonomous braking but being an extended feature of ACC, I can't see why it should have an upper speed limit. I haven't used autonomous braking (ACC must be engaged) to bring the RR to a stop, only down to about 20 kph in slowing traffic before taking control.

I had just begun to change my line slightly to give the stopped car a wider berth as I have done on several occasions when FA has activated whilst travelling above urban speeds but this was the 1st time that ACC had been engaged so I didn't have in mind that the RR would try to stop, hence the surprise. I'm happy that the RR followed the rules and tried to stop but I will have to be careful to disengage ACC in such circumstances, as I do if traffic ahead suddenly slows.