Crash Systems | Autonomous Emergency Braking
Mercedes-Benz’s PRE-SAFE® is a system intended to bridge the gap between primary safety, which aims to prevent a car from being involved in an accident, and secondary safety, which is the protection provided during a collision. At speeds above 30km/h, PRE-SAFE® monitors the dynamic state of the vehicle (speed, rotation etc.) and the driver’s inputs to steering, accelerator and brake, to determine whether or not emergency action is being taken. If so, the system deems that a collision is imminent: among other measures, it takes the slack out of the seat belts using reversible tensioners; it optimises the occupants’ seating position if electrically adjustable seats are fitted; and, if there is much rotation or side slip and a side impact or roll-over is considered likely, it closes the electric windows and sunroof. PRE-SAFE® is always on; it cannot be switched off by the driver and provides protection at all speeds above 30km/h.
All of the actions taken by PRE-SAFE® are reversible: if the collision is avoided, tension is removed from the seat belts and the occupants can readjust their seats. .
What is the safety benefit?PRE-SAFE® is designed to provide protection in a broad range of accident types. However, the actions it takes (e.g. tensioning seatbelts, adjusting seat position) are likely to be most effective in frontal collisions and in side impacts.
PRE-SAFE® operates at speeds above 30km/h and is focussed at providing protection in accidents which are more severe than those typical of urban driving. The anticipated benefit is biased towards severe and fatal injuries. It is estimated that nearly five percent of all casualties in accidents involving a passenger vehicle could be addressed by PRE-SAFE®. If all cars were equipped with PRE-SAFE®, this would be equivalent to the system being activated, and providing some measure of protection, in some 55,000 injuries in EU 27 each year.
How the system assessed?
Analysis was done of the forward movement of occupants during heavy braking and the effect of removing slack from the seatbelt was established. By looking at crash tests using different dummy positions, the effect of optimising occupants’ seating position before the start of the impact could then be determined. Finally, Mercedes-Benz’s accident research group carefully monitors the real-world crash performance of its vehicles and has extensive data on the performance and effectiveness of its advanced safety features.
PRE-SAFE® does not work autonomously; it depends on some action by the driver in order to identify when an accident is likely. In this regard, the system offers no benefit in accidents which the driver has not himself predicted. The system has to achieve a balance between sensitive triggering, offering a high safety benefit, and the irritation caused to drivers if e.g. seatbelts were regularly tightened, when there was no real danger. Daimler has optimised the system to achieve this balance.
PRE-SAFE® is rewarded for its availability on the Mercedes-Benz E-Class, on which it is fitted as standard equipment in all European countries. The system is also available on other vehicles which have been tested by Euro NCAP, such as the Mercedes-Benz C-Class, GLK-Class, M-Class. The availability of optional equipment may vary from country to country. In some countries, optional safety equipment may be available only when combined with other features as part of a package, and may not be offered on all variants. You should check with your dealer to see if the system is available to you.
Mercedes-Benz’s basic PRE-SAFE® is a system which, at speeds above 30km/h, uses the driver’s inputs to steering, accelerator and brake, as well as dynamic data about the speed and rotation of the vehicle to determine whether or not emergency action is being taken, or if the car is out of control. If so, among other measures, the system takes the slack out of the seat belts using reversible tensioners and, if a side impact or roll-over is considered likely, it closes the electric windows and sunroof.
PRE-SAFE® Brake is an autonomous emergency braking system which additionally uses radar sensors to help identify critical situations. At speeds between 30km/h and 200km/h, a distance some 200m ahead of the car is scanned for radar reflective obstacles. Some two and a half seconds before impact, the driver is warned of a potential danger. If, at that stage, the driver applies the brakes, the car automatically delivers the brake force needed to bring the car to a safe stop (if that is physically possible), regardless of the pressure applied by the driver. However, if the critical situation continues to develop and the driver does not react, the car applies partial braking around one and half seconds before the collision and tensions the seat belts in preparation for the impact. Finally, if PRE-SAFE® Brake determines that a collision cannot be avoided, the car applies maximum braking force to reduce the speed of the impact as much as possible.
What is the safety benefit?It is estimated that around 19 percent of all serious accidents in Germany are rear end collisions in longitudinal traffic. The main causes of such accidents are driver distraction or inattentiveness, and driver misperception where a driving situation is misjudged. PRE-SAFE® Brake operates at speeds between 30km/h and 200km/h. The system is therefore focussed on high speed accidents rather than those typical of urban driving, and the anticipated benefit is biased towards severe and fatal injuries. A decrease of 35 percent in the numbers of those who are at least severely injured is thought possible in cars equipped with PRE-SAFE® Brake. If all cars on the road were equipped with such a system, this would be equivalent to PRE-SAFE® Brake providing some measure of protection in accidents where, currently, around 6,500 people are severely or fatally injured in EU 27.
How the system assessed?
In addition to the work done for the basic PRE-SAFE® system to determine the influence of seatbelt tension, extensive track testing was done to ensure that PRE-SAFE® Brake met targets for when warnings were given to the driver, and the points at which partial and full braking were applied. These track tests established the degree to which the impact speed was reduced in different accident scenarios against a simulated ‘target’ vehicle. Mercedes-Benz’s accident research group carefully monitors the real-world crash performance of its vehicles and has extensive data on the performance and effectiveness of its advanced safety features.
PRE-SAFE® Brake can be switched off by the driver. If it switched off, it remains off until it is turned back on again and does not default to ‘on’ at the beginning of a new journey.
PRE-SAFE® Brake complements the basic PRE-SAFE® system to help predict when an accident is likely to happen. To do this, PRE-SAFE® Brake requires a sufficient radar reflectance of objects which the car is approaching. Also, the radar sensor needs a clear ‘view’ of the road in front and its efficiency is compromised by contamination such as snow, mud or leaves.
PRE-SAFE® Brake is rewarded for its availability on the Mercedes-Benz E-Class, on which it is available as an option in EU 27 as part of a safety package. You should check with your dealer to see if the system is available to you.