Braking Indicator

  Has this been done? Seems like a pretty obvious idea so I guess it has, but been thinking of this for years and never seen it yet. Seems like a real good usp for a SAAB or Volvo. A light which is off the shelf to buy, battery powered, retails for around £30 and without tampering with the cars machanics at all will alert drivers behind of the proportional amount of breaking and de-acceleration. Perhaps a system of LED'S or amount of light intensity.
  Any comments??