BAS is essentially the result of interesting discoveries made by passenger car development engineers during their analyses of accidents. The engineers found that in many cases drivers reacted correctly and quickly enough in critical situations. For example, they tended to step on the brakes hard and fast when impact seemed imminent. However, the researchers also discovered that the drivers often applied too little force. For instance, in the case of an imminent impact, drivers frequently failed to engage the full power of the braking unit. The result was often a collision with the vehicle in front. BAS prevents this from happening. Its electronic system interprets the rapid pressure applied to the brakes as an attempt to implement an emergency braking maneuver. It then generates maximum braking pressure, regardless of how forcefully the driver actually engages the pedal.

Brake Assist PLUS uses two radar systems. Both of these radar systems are very good at not only registering objects up ahead but also calculating the distance from these objects and their relative speed. If the distance between the vehicle and the registered object is so small that there is a real danger of collision, Brake Assist PLUS issues a visual warning on the instrument cluster display and sounds an alarm over the car speakers. While this is happening, the electronic system calculates the braking deceleration necessary to avert the collision. If the driver then hits the brakes, the braking pressure needed to bring the vehicle to a stop before impact is immediately provided automatically. Ensuring that an appropriate braking force is not exceeded gives those driving behind more time to react.

The PRE-SAFE® Brake for passenger cars goes a step further and brakes the vehicle if an acute risk of a head-to-tail collision is detected and the driver fails to react to the warnings.

Mercedes-Benz has continued to further develop the PRE-SAFE® Brake, which was first unveiled in 2006. From spring 2009, a version of the system that activates the maximum braking power automatically if there is an acute risk of an unavoidable accident will be available for the new E-Class and the S-Class. The PRE-SAFE® Brake therefore acts as an electronic crumple zone and can greatly reduce the severity of an impact.

What's more, it does this in two stages: around 1.6 seconds before the calculated impact point – after three audible warning signals – the system initiates partial braking autonomously and decelerates the car with around 40 percent of the maximum braking power. Designed to supplement the visual and audible warnings, autonomous partial braking gives the driver a further, perceptible signal to act. If the driver then brakes immediately, the maximum braking force is made available or, if the driver swerves, the accident can be avoided at the last moment, depending on the driving situation.