Emergency brake assist systems help to prevent rear-end collisions of trucks, for example in freeway traffic jams. The combined radar and camera technology from Knorr-Bremse reduces false alarms from the technical system and brings the vehicle to a standstill if the driver does not intervene after a warning signal.
On the A11 heading north from Berlin towards Stettin, in the midday sun, the radio brings the news. Peter Maisels allows his thoughts to wander as he sits at the wheel of his articulated truck: "How much forest burned recently in Mecklenburg-Western Pomerania? Over 1,000 hectares?" Maisels tries to visualize the number, looks out to the dry landscape on his right. Suddenly a warning signal beeps, quickly followed by a jolt of his driver's cab. Maisels' head is thrust forward. The traffic ahead is at a standstill, but there is still a safe gap before the vehicle in front. Maisels can switch from the accelerator to the brake pedal, check the situation behind him and switch on his hazard lights to avoid any collisions from the rear. Before his truck comes to a complete stop, he reaches for his thermos bottle with his right hand. Fortunately, it has stayed in its holder.
First partial, then emergency braking
Maisels' emergency brake assist had already sounded a warning around 80 meters away from the traffic jam. But in that second the driver was busy thinking about the forest fire. That's why the emergency braking system automatically activated partial braking. "Our brake assist starts braking from a good 50 meters away if the distance to the object in front continues to decrease," explains Dr. Frank Leitner, the responsible Product Manager at Knorr-Bremse’s Commercial Vehicle Systems division. There is no emergency braking right away, but the deceleration of one to two meters per second squared is enough to "noticeably rock the cab forward". "If the driver still doesn't react, our emergency brake assist will perform a deceleration to zero starting from a distance of 30 meters." Huge forces are at work here. On a dry road, a truck decelerates at around eight meters per second squared. It's roughly the same as a sports car that takes 4.6 seconds to go from 0 to 100 km/h – except it's slowing down. The force distributed over the entire backrest when a sports car hits the gas is absorbed by the seatbelt alone when braking. What's more, in a truck it feels like the 40 tons continue to push the vehicle forward. "When approaching a stationary obstacle, even experienced drivers experience a moment of stress," says Leitner.
When approaching a stationary obstacle, even experienced drivers experience a moment of stress.
Dr. Frank Leitner – Product Manager Automated Driving at Knorr-Bremse’s Commercial Vehicle Systems division
Emergency brake assist in the eyes of the law
Since November 2015, emergency brake assist has been mandatory in Europe for newly registered vehicles. However, drivers had the option of turning it off for the entire journey after ignition. From a current standpoint, the required deceleration appears rather minimal: a speed reduction from 80 km/h to 70 km/h before impact. In 2018, Brussels tightened its regulations, although the law still allows the vehicle to be travelling at 60 km/h upon impact.
Combination of systems for error-free detection
Leitner: "Safety barriers on a curve, a slightly sloping road beneath an overpass, possibly with large road signs attached, roundabout islands when entering or exiting a town, bridge piers – they all have to be reliably distinguished from real obstacles on the road by the emergency brake assist technology." Then there are vehicles changing lanes: they may briefly violate the distance limits, but usually move on continuously. "Combined emergency brake assists like ours bring together video imaging and radar sensors, allowing them to identify such phenomena with virtually no errors. They compare the information from both sources in real time to eliminate typical technical errors, continuously calculate the change in distance and – road conditions permitting – brake to a standstill if the driver does not intervene," says Leitner.
Drivers have the best overview
As soon as the driver intervenes by firmly applying the accelerator, the automatic systems disengage and return full control to the driver. During the warning and partial braking phases, the driver can also regain full control by steering or braking. For Klaus Bogumil this is only logical. "The assistance systems can certainly help in everyday situations," says the Fleet Manager at Wormser Qualitätslogistik. In fact, he can remember an accident where an emergency braking assist could have made all the difference. However, "drivers themselves are best at seeing the overall picture when the case is not clear-cut," Bogumil is convinced. "For example, drivers can also see what's happening behind their truck, which can be crucial when deciding whether to switch lanes," he explains.
Correcting human errors with automatic emergency braking
Frank Leitner also stresses that "the driver is always in charge. Why should the system decelerate to zero as the end of a freeway entrance ramp approaches if the driver can see that the road is clear behind the car on the left and a lane change is safe as soon as it has passed?" However, he also believes that the emergency brake assist can reliably correct errors made by a human driver. For example, if the driver steers to avoid the original obstacle, but the adjacent lane is occupied. Normally, the automatic braking system ceases to detect the obstacle and stops braking. Leitner: "Our multi-lane system also continues to brake when the adjacent lane is blocked by another object – until the vehicle is stationary." Another moment that would have Peter Maisels reaching for his thermos flask!