It's the legislation that gets in the way of implementing an effective brake light system to work in tandem with regenerative braking.
It has already been said that dropping a cog or two and leaving your foot off the accelerator can have a dramatic deceleration effect. In my opinion, the problem with brake lights is that they are either on or off, there is no indication of the severity with which the car is braking. If I had things my way, the brakelights in the light clusters would be either on or off depending on the brake pedal. However, I would make the high-level light as one long LED light bar, controlled by a decelerometer, programmed such that more LEDs are lit the more intense the braking (or other means of slowing down). By having two seperately activated systems, you increase the reliability of the system, as well as indicating deceleration by other methods (including regenerative), and giving the driver behind an indication of how hard to brake. Would stop most of the suicide braking on the outer lane of the M6 for sure.
Now of course, my idea would never pass legislation, that's why I am happy to air it publically since it would never make me any money.