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I did a course that included this system on BMW and it is surprising you have to be foot down no reaction for it too work , we had a cardboard cut out stood in the car park we had to drive at and he got run over more than we stopped because, we knew and could see it when driving at it you think this isn't going to stop in time, the second you react by lifting the throttle off the ecu thinks no need for me they have seen it and bang under the car it goes but be brave keep your foot planted and the car slams the brakes on so hard if you didn't have a belt on you would be kissing the screen :D
I'm not sure whether my car auto-brakes or not if there's a collision imminent. Not really got a good way to test it.