Yesterday, on my way home to Berkshire on A 303 I've experience the same problem, but with more serious consequence, had the car IS 220d model crushed into a roadside pylon.
Was on cruise control at the same 50 mph speed as you (what a strange coincidence?!) when approaching a slower moving queue slightly hit the brake to adjust the speed to the traffic ahead when I had the surprise to realize that it was not working, tried several times to hardly hit the brake pedal but no sign of slowing. In panic I forgot about the hand brake that would have helped me to stop the car, eventually tried to shift to a lower gear and as a result while sliding through the narrow corridor of ongoing and oncoming traffic, eventually scratching an old laddies car and hitting the next one in the right rear corner I saw a gap and turned the steering sharp left into the roadside line of bushes. Ultimately the car crushed approximately at the same speed speed I was cruising before into an electricity pylon in the middle of those bushes. What a dreadful experience, but lucky nobody was injured, except me a little. All 4 front airbags deployed (thank God those did not fail anyways). That maybe saved my life, I realize now, because, as I said, the car did not slow an inch while I was hitting the brakes, maybe just a little from the impact with the car driving in front of me right before I found that bloody pylon.
Now I am still in kind of shock and really cannot figure it out what happened then, because no warning light on dashboard concerning ABS or barking, the braking fluid is alright. Seemed to me more like an electronic issue at the moment. I am really out of my spirit. Those cars are not supposed to behave like that.
