Press Release
Automatic partial braking before impending rear-end collision
- System intervenes if driver does not react in face of acute danger of acci-dent
- Partial braking prompts driver to act in order to prevent accident
- Significantly reduces severity of accident where collision cannot be avoided
- Logical supplement to the anticipatory Brake Assist PLUS system
- PRE-SAFE® occupant protection automatically activated on partial braking
Seconds – sometimes even fractions of a second – make the difference between an accident happening and its being averted. In these critical moments it is crucial both to recognise the danger in good time and to act quickly and decisively to avoid a collision. The cutting-edge technology of the S-Class can provide effective support for the driver on both counts, significantly improving road safety.
On recognising a potential accident situation, help is provided by two near-range radar sensors behind the front bumper and a long-distance radar in the radiator grille. These permanently record the traffic situation in front of the S-Class. The near-range radar (24 gigahertz) operates with a range of 30 metres and an effec-tive angle of 80 degrees, while the long-distance radar (77 gigahertz) has an open-ing angle of nine degrees and is capable of scanning a three-lane, 150 metre-long stretch of motorway. Computers assess the data from both radar systems. If the distance between the S-Class and the car in front is too small, a red warning sym-bol lights up in the instrument cluster.
If the vehicle in front, registered by the radar, brakes suddenly, creating the risk of a collision, an additional warning tone also sounds. This is an unambiguous prompt for the driver to brake. Assistance is provided by Brake Assist PLUS (BAS PLUS) – as soon as the first warning tone sounds, the system automatically de-termines the brake pressure required in this situation to prevent the collision. In other words, the system helps the driver to recognise the risk and makes the cal-culated brake boosting force instantly available, even if the driver does not step on the brake forcefully enough. Depending on the speed and distance, BAS PLUS therefore allows controlled, targeted braking. If necessary, the system increases the braking force up to full braking. Wide-ranging tests of the effectiveness of Brake Assist PLUS carried out in the simulator show that this system reduces the accident rate in typical situations involving the risk of a rear-end collision from 44 to 11 percent. It should be noted, however, that support is only provided by BAS PLUS if the driver gives the braking command – by stepping on the brake pedal.
Autonomous PRE-SAFE® partial braking with deceleration of up to 0.4 g
Unfortunately, practice shows that drivers do not always react as quickly as is needed at critical moments – for example because they are distracted and fail to register the warning signals provided by Brake Assist PLUS. The newly developed PRE-SAFE® brake intervenes in situations such as these, automatically braking the S-Class if there is acute danger of an accident. The Mercedes experts call this “autonomous partial braking”, an automatic braking process with deceleration of up to 0.4 g (approx. four m/s²), in other words around 40 percent of the maximum braking performance.
Just how quickly the S-Class’s assistance systems recognise a potential accident situation, warn the driver and provide driver assistance or, if the danger is acute, intervene themselves, can be seen by looking at a timeline representing a typical rear-end collision situation:
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Around 2.6 seconds before the moment of impact calculated by the system the audible warning signal sounds. A red warning symbol in the tachometer also informs the driver that there is a danger of an accident.
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Approximately 1.6 seconds before the calculated accident – if the driver has not reacted to the three acoustic warning tones, the PRE-SAFE® brake carries out autonomous partial braking.
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Around 0.6 seconds before the impact the driver has a final chance to avert the accident by swerving rapidly or applying full braking. This means that, after the PRE-SAFE® brake has intervened, the driver has around a second to act.
The PRE-SAFE® brake is active in the speed range between 30 and 180 km/h, where traffic is registered in front of the car. The system also reacts when approaching a stationary queue of traffic, provided the S-Class is not travelling at over 70 km/h.
Aversion of accident or significant reduction in severity of accident
Autonomous PRE-SAFE® partial braking has a twofold effect: the intervention of the PRE-SAFE® brake makes distracted drivers aware of the situation, alerting them to the acute danger and indicating that immediate action is required if the accident is to be averted. The driver is further aided in the attempt to avert the accident by the S-Class’s safety systems: on emergency braking, Brake Assist PLUS instantly calculates the optimum braking force for the given situation; if the car veers rapidly to one side, ESP® stabilises the Saloon and reduces the risk of skid-ding.
If the collision cannot be avoided, the PRE-SAFE® brake can help to reduce the consequences of the accident. In comparative tests in the simulator, where participants drove with and without the new system and were deliberately distracted, the impact speed was reduced by autonomous partial brak-ing from an average of 45 to 35 km/h. This means 40 percent lower crash energy and with it a significantly reduced risk of injury for driver and front passenger. Impact-induced stress to the upper body is reduced as a result by up to 20 percent. In other words, in a rear-end collision the PRE-SAFE® brake acts like a virtual crumple zone, dissipating impact energy even before the crash.
Ride simulator: 70 percent of all test drives remained accident-free
Accident rates in extensive simulator tests involving deliberate distraction of the driver’s attention underline the effectiveness of the Mercedes-Benz safety systems in the S-Class:
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The majority of the 70 participants reacted spontaneously to the visual and audible prompts from the S-Class’s proximity control system and were able to avert the accident with support from Brake Assist PLUS. As a result, 53 percent of the test drives remained accident-free.
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17 percent of participants only reacted when autonomous partial braking occurred and then stepped on the brake quickly enough to avert the accident with the aid of the PRE-SAFE® brake and Brake Assist PLUS.
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One third of drivers were so distracted by the scenario on the opposite carriageway that they did not manage to brake in time. In these cases the PRE-SAFE® brake significantly reduced the impact speed – and hence the severity of the accident.
Automatic activation of pre-emptive occupant protection
The new system is set to become a key part of PRE-SAFE® occupant protection, a standard feature of the S-Class. It utilises the precious, previously unused moments between recognition of the risk of an accident and activation of the protection systems in the interior with the aim of further enhancing occupant safety. Because PRE-SAFE® is networked with the ABS, Brake Assist and ESP® sensors, the system recognises critical dynamic handling situations such as serious skidding or heavy braking and processes this sensor information to prepare the occupants of the car for an accident by way of precaution:
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On emergency braking the front seat belts in the S-Class are pre-emptively tautened and air chambers in the dynamic multicontour seats (available as an option) are inflated, supporting the driver, front passenger and rear passengers and holding them more securely in their seats. In addition the front passenger seat is moved into the optimum position with regard to fore-and-aft lo-cation and the angle of the seat cushion and backrest, ensuring that the belt and airbag are able to operate to best effect. In the rear of the S-Class PRE-SAFE® corrects the backrest angle and seat cushion setting. The rear head re-straints are also raised.
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If there is a risk of skidding, the system also begins to close the S-Class’s side windows and the sliding sunroof. This means, for example, that in the event of a side impact or a rollover the windowbags are better supported. This PRE-SAFE® measure also reduces the risk of occupants being flung out of the open side windows or sliding roof or of objects penetrating the interior.
The new PRE-SAFE® brake helps refine this precautionary occupant protection system, as many of the above pre-emptive measures will in future be activated if the driver has not reacted to the warnings from the cockpit and has not braked the vehicle despite the risk of accident. As soon as the system begins autonomous partial braking, the front belts are automatically tautened, the support cushions in the dynamic multicontour seats inflated and the front passenger seat moved to the optimum position.
