I would imagine it would only switch the brake lights on if it were using the brakes. Otherwise it would help bring our motorways to an even greater state of gridlock than they already are!
When the motorway is at that certain density of traffic, it only takes one heavy handed tailgater's brakes to rattle through a domino-effect of braking that ripples backwards through the column of traffic, each car applying more and more braking until eventually the whole thing has ground to a halt.
It's probably why the inside lane tends to operate better in such conditions - lower incidence of tailgaters and 'this is my road' charlies*
(*technical term

)