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Emergency stop signal (ESS)

Toyota, Mercedes-Benz, Volvo, and BMW have released vehicles equipped to convey a special light signal when the vehicle is braked rapidly and severely. This is officially referred to as "emergency stop signal", and UN Regulation 48 calls for the lamps providing the ESS to flash at 4 Hz when a passenger car decelerates at greater than 6 m/s2 or a truck or bus decelerates at greater than 4 m/s2. Mercedes vehicles flash the stop lamps for the ESS, while vehicles from the Volkswagen Group of manufacturers (VW, Audi, SEAT and Skoda) flash the hazard flashers.

Other methods of severe-braking indication have also been implemented; some Volvo models make the stop lamps brighter, and some BMWs have "Adaptive Brake Lights" that effectively increase the size of the stop lights under severe braking by illuminating the tail lamps at brighter-than-normal intensity. As long as the brighter-than-normal stop lamps are within the regulated maximum intensity for stop lamps in general, this kind of implementation does not require specific regulatory approval since the stop lamps are under all conditions operating in accord with the general regulations on stop lamps.

The idea behind such emergency-braking indicator systems is to catch following drivers' attention with special urgency. However, there remains considerable debate over whether the system offers a measurable increase in safety performance. To date, studies of vehicles in service have not shown any significant such improvement. The systems used by BMW, Volvo, and Mercedes differ not only in operational mode (growing vs. intensifying vs. flashing, respectively), but also in such parameters as deceleration threshold of activation. Data are being collected and analyzed in an effort to determine how such a system might be implemented to maximise a safety benefit, if such a benefit can be realised with visual emergency braking displays. An experimental study at the University of Toronto has tested stop lights which gradually and continuously grow in illuminated area with increasing braking.

One potential problem with flashing stop lamps in the United States (and Canada) is the regulations that permit flashing stop lamps to be used in lieu of separate rear turn signal and hazard warning lamps.