We were reminded of the incredible goodness of the three-stage sequential turn signals used by late-60s/early-70s Mercury Cougars after seeing the '73 Cougar DOTS car a week ago, so a little digging on the Tube That Is You and voila! The mechanism behind the magic was actually a failure-prone motor-driven mechanical device, apparently derived from pinball-machine technology, but the many-moving-parts analog beauty of it all just makes this feature that much cooler.
@Turboner: the BMW system is actually even smarter than that. It uses accelerometers to measure the deceleration, and if the G-force crosses a threshold it kicks in extra brake-LED clusters.
That said, I've only ever noticed it while following a Beemer once, and I knew what I was looking at. I don't know if most folks would notice, or interpret it as emergency braking. The flashing Mercedes idea sounds like a better implementation.
I also recall that it was like a £600 option (~$1200) on an E46 3-series. And since getting rear-ended in the UK is always the other party's fault, insurance-wise, I'm not so sure what the incentive is.
@JoeRenault: +1 on all that. And don't forget the diesel thing. Lazy inefficient low-mpg V6's and V8's in everything? Fine! A few particulates that the rest of the civilised world is OK with? Someone think of the children!