Uhhhh...guys, the part about automatic braking is already here...
Several models that offer automatic collision avoidance packages that include automatic slowing based upon speed and vehicle closure rates are available.
Trust me, with the way the traffic patterns in major cities are, you are going to see "black box" controls that limit speed in certain areas within the next 10-15 years, maybe sooner. In fact, you may see systems that automatically route you based upon congestion within that time, and I'm not talking about voluntary rerouting based on your desires, but on the preconfigured routing some "traffic specialist" configures based upon traffic flow and rate of speed.
Forutnately, I live WAY outside any city that might do this, but in the "configurable" traffic options that some city/highway planners are considering, none of the above is out of the question.