TelmoF:
and a deceleration detector, which will activate the brake light automatically
What will cause the deceleration apart from your foot pressing the brake pedal? AFAIK the brake light is already operated by pressing the brake pedal. And I believe the pedal switch actually operates the light fractionally before the brakes are applied which gives following traffic a little more warning time compared to a system that only operates after the brakes have been applied long enough, and sufficiently hard to trigger a deceleration sensor.
On the other hand, false brake indications just caused by gentle deceleration (when the throttle is released but the brakes are not applied) will just confuse and irritate following traffic. A blinking light will be assumed to be an indicator.
Road safety relies to a large extent on long-established predictable behaviour (so people can apply what they learned in driving school) and new gizmos that are not widely publicised are more likely to cause problems than to solve them.
...R