In 1998 a long-held dream for many drivers was fulfilled in the S-Class with the DISTRONIC autonomous intelligent cruise control system: an electronic co-pilot which keeps the car at a set speed and at a set distance from the vehicle ahead – comfortably and stress-free. Mercedes-Benz was the world’s first automobile manufacturer to offer its customers such a system.

Autonomous intelligent cruise control – this term revealed the nature of this innovative system, namely a logical further development of the cruise control system which had long proved its worth as a convenience function on less busy roads by keeping the vehicle to a speed preset by the driver. Once traffic becomes more dense, with a constant reduction in the gap between vehicles, the driver is however obliged to intervene personally, change the preset speed or switch the cruise control function off altogether.

It was for such traffic situations that Mercedes-Benz developed DISTRONIC. This combines the cruise control function with a radar sensor in the radiator grille which monitors the traffic situation ahead of the S-Class up to a range of 150 metres. For this purpose the system uses one of the fastest and most powerful micro-computers ever developed for automotive applications. This processes the signals from the radar sensor, which are reflected by vehicles ahead and change their frequency. The control unit uses this change in frequency to calculate the relative speeds of the two vehicles, and determines the distance on the basis of the time taken for the signals to reflect.

If the gap between the vehicles decreases, the proximity control system automatically decelerates or operates the brakes if necessary. If the vehicle ahead increases the distance or changes lane, DISTRONIC operates as a conventional cruise control system and keeps the speed to the preset level.

Further development: two radar systems for long and short range

Mercedes-Benz has developed this highly efficient support system even further for the new S-Class, hence the name DISTRONIC PLUS.

By combining the previous DISTRONIC radar (77 GHz) with the newly developed close-range radar (24 GHz), which is also used for Brake Assist PLUS, Mercedes engineers have been able to extend the working range of the proximity control system even further, from 0.2 to 150 metres. While the 77-Gigahertz radar monitors the road ahead over a long distance, the signals from the 24-Gigahertz sensors have a range of around 30 metres and monitor the immediate vicinity of the vehicle. The electronic control unit analyses the information received from both radar systems and calculates the engine, automatic transmission and braking parameters required for proximity control.

In addition, the speed range for this control function has been increased from the previous 30 - 180 km/h to zero - 200 km/h. This means that when the traffic tails back, DISTRONIC PLUS automatically brakes the S-Class to a standstill and accelerates back to the set speed as soon as the traffic situation allows. To move off automatically, the driver only needs to operate the DISTRONIC stalk on the steering column or briefly depress the accelerator.

As before, the required distance can be set according to the traffic flow and density: using a control function on the DISTRONIC stalk, the driver is able to variably adjust the time lapse between one and two seconds if required by the traffic situation. The medium setting corresponds to a time lapse of 1.5 seconds, representing a gap of around 42 metres at a speed of 100 km/h. The target and actual distances between the S-Class and the vehicle ahead are shown in the central display of the instrument cluster.

Head-to-tail traffic: automatic braking to a standstill

As more dynamic proximity control intervention is often required at lower speeds, i.e. in stop-and-go traffic with frequent starting and braking, the acceleration and deceleration values of the system have been reconfigured. Depending on the speed, automatic deceleration of up to four m/s² (previously two m/s²) is now possible. If DISTRONIC PLUS detects that heavier braking is required, a warning light appears in the instrument cluster. This is accompanied by an acoustic warning which tells the driver to watch the traffic situation and apply the brakes himself if necessary.

The electronic proximity control system can be activated independently of DISTRONIC PLUS. In this case too, the system continuously monitors the distance from the vehicles ahead and gives the driver an acoustic warning if he is approaching another vehicle too rapidly at a speed of more than 30 km//h, and lights up a warning lamp if the gap is too small.

The DISTRONIC PLUS equipment package also includes the preventive Brake Assist PLUS and the new Parking Assist.

Practical trials: DISTRONIC as a contribution to greater safety and reduced stress

Mercedes-Benz examined the positive effects of DISTRONIC on driving and driver-fitness safety during long-distance trials in the USA and Germany. More than 140 male and female drivers took part, covering a total of almost 200,000 kilometres. The results:

• At 70 to 110 km/h the safety gap increased by an average of 29 percent when the test drivers had activated DISTRONIC. In the USA this support system increased the gap by a mean of 13 to 25 percent.

When DISTRONIC was switched on, the style of driving was smoother and therefore more relaxed than without the system. This was shown by the measured acceleration and deceleration times.

When the proximity control system was activated, the mean heart rate of the test drivers only increased by an average of 1.8 beats per minute compared to the normal pulse. When driving without DISTRONIC the heart rate increased by 3.2 beats per minute, which indicates more stress on the drivers.

Mercedes-Benz also subjected DISTRONIC PLUS to exhaustive practical trials. These involved more than 200 male and female drivers in Europe and the USA, who thought very highly of this improved support system. This was illustrated by frequent use of DISTRONIC PLUS during the long-distance journeys: while the conventional DISTRONIC system was "only" switched on for around one third of the routes in Germany, DISTRONIC PLUS was in action for more than half of all journeys. In the USA the drivers switched on the new system for as much as two thirds of the routes, while the rate of use for the previous proximity control system was around 42 percent.