Daimler HighTechReport: The Fascination of Technology - Issue 02/2008 - PART IV


Stuttgart, Nov 25, 2008

Bus Safety: All-Round Safety - Travego with Active Brake Assist and Front Collision Guard

When Katrin Breitrück decided to study Aerospace Engineering at the University of Stuttgart, she never could have imagined that her career would one day focus on road vehicles. That’s what happened, though, and Breitrück now works as an engineer at Daimler Research and Advanced Engineering, where she heads the Active Brake Assist project. Thanks to the emergency braking system, bus travel could become even safer.

Active Brake Assist’s premiere at the IAA Commercial Vehicle show in Hanover, Germany, in the fall of 2008 made Mercedes-Benz the world’s first manufacturer to introduce a proactive emergency braking system for buses. Driving presentations with the brand’s Travego model have shown how Active Brake Assist helps prevent rear-end collisions, thereby saving lives.

Breitrück explains how the system works: “When there’s an imminent danger that the bus is going to hit a slower moving vehicle in front, Active Brake Assist issues a warning to the driver, initiates a partial braking maneuver, and, if necessary, engages a full braking maneuver.”

Proven system Active Brake Assist has been available in the heavy-duty Mercedes-Benz Actros truck since 2006. During this time, the system has proved its worth in customer operations over a total distance of 350 million kilometers. It wasn’t possible for Daimler engineers to simply transfer Active Brake Assist as it was to buses, however. “For one thing, buses move faster and display different handling characteristics,” says Breitrück. “In addition, we also had to take into account the fact that with buses you’re also dealing with passengers.”

Active Brake Assist (ABA) is based on the proximity control system that has been available in Mercedes-Benz trucks and buses for several years. ABA uses the proximity control system’s radar sensor, which is mounted in the front of the vehicle, behind the Mercedes star. The sensor utilizes three radar beams to probe the area in front of the bus at 50-millisecond intervals. It continually measures the distance to, and relative velocity of, any vehicle ahead of it. “The ABA control unit calculates in real time whether a critical situation is in the making,” Breitrück explains. “That would be the case, for example, if the bus in question was traveling at 80 kilometers per hour but the vehicle in front was only doing 40.”
Warning If the bus driver fails to notice that he or she has gotten too close to the vehicle in front, and an accident thus appears unavoidable, ABA steps in by initiating a multi-stage warning process. First of all, it warns the driver with a red triangle that lights up on the instrument panel and displays the contours of a vehicle. In addition, the system simultaneously emits a beeping sound and turns off the hands-free telephone function and the radio.

Partial braking If the danger of a collision becomes more threatening because the driver has failed to react to the initial warning, ABA implements a partial braking maneuver. “This takes the form of light braking, whereby brake pressure increases linearly up to a maximum of 30 percent of the total possible braking force,” Breitrück explains. Bus passengers feel this braking maneuver, and this gives them a chance to hold on to something if they happen to be standing. Truck drivers are always sitting and buckled up, however, which is why ABA in trucks forgoes the partial braking stage and immediately initiates emergency braking instead.
Emergency braking During the automatic partial braking maneuver, the red triangle remains lit up and the beeping sound continues as well. If the driver still fails to react, Active Brake Assist moves to prevent a serious accident by automatically applying the full braking power, leading to an emergency stop. The beeping sound then becomes a permanent warning tone.

ABA doesn’t just warn the bus driver, however. The system also alarms vehicles behind the bus by activating the brake lights, which light up normally during the partial braking maneuver - as if the driver were doing the braking. The brake lights flash whenever the full braking power is applied, thereby indicating the dangerous situation at hand. When the bus is brought to a stop by emergency braking, ABA keeps the brakes locked for a maximum of three seconds and automatically switches on the hazard warning lights.

Although Active Brake Assist automatically initiates the emergency braking maneuver, the driver remains in complete control of the situation and can override the electronic assistant at any time. “The multi-stage warning concept ensures that the driver always has the possibility to swerve or brake as needed to avoid a collision,” says Breitrück. As a result, if the driver activates the turn signal lights, hits the gas, or brakes, ABA will turn off the warning signal and cancel the command for the partial braking maneuver.

ABA is always activated when the engine is turned on, after which its control unit carries out a redundant calculation in real time and continually checks to see if the driver is braking, accelerating, or using the turn signal lights in a particular moment. In such cases, the system remains on but inactive, and the driver can in fact deactivate it at any time with just the push of a button. Engaging this switch - but also when kickdown occurs during acceleration - will deactivate ABA in any of its phases, including the emergency braking stage.

Safety In order to guarantee Mercedes-Benz’ high safety standards in ABA for buses, Breitrück and colleagues from Research and Advanced Engineering, as well as from the Buses unit, put the system through its paces from the very beginning. Initial software tests on computers were followed by examinations of the software’s functioning in the control unit, with the latter subsequently tested with regard to its interaction with other electronic components in the E/E system. It was only after these extensive series of tests that the experts installed the new system in a bus, where it was then tested even further. “This multi-stage testing approach and close cooperation with Daimler Research, Truck Development, and the Buses unit enabled us to ensure the requisite high quality,” says Breitrück.

Accident statistics Active Brake Assist can’t always prevent an accident - but its initiation of an emergency braking maneuver does significantly lower the speed at which the bus is traveling should a collision occur, thereby reducing the severity of the accident. A look at accident statistics reveals that ABA covers an additional area of danger - that of rear-end collisions.

The two most frequent causes of severe bus accidents are drivers drifting out of the lane they’re traveling in and buses tipping over. Mercedes-Benz has two systems that help prevent such things from occurring: the Lane Assistant and the Electronic Stability Program (ESP). The third most frequent cause of bus accidents are rear-end collisions. While these rarely happen, they are among the most serious. “We systematically tackle the causes of real accidents as part of Mercedes-Benz’ integrated safety concept, and in this sense, ABA marks a very important step on the road to providing greater safety for both drivers and passengers,” says Richard Averbeck, Managing Director of EvoBus Development (see page 18, “90 Seconds with…”).
Front Collision Guard Another important contribution to greater safety will be made by Front Collision Guard, which celebrated its world premiere with ABA in the Travego. “Front Collision Guard is a unique passive bus safety system that protects the driver and tour guide in the event of a frontal collision,” says EvoBus engineer Gregor Steinmetz, who managed the development of the system.

Put in simple terms, Front Collision Guard (FCG) is a specially designed support and body structure integrated into the front of the bus, whereby a stable transverse profile is mounted behind the front bumper. This profile acts as an underride guard that can prevent a car from being squeezed underneath the coach, for example. Behind the profile is an impact-absorbing frame consisting of crash elements that absorb energy in a predetermined manner in the event of a severe frontal impact.

The third element of Front Collision Guard is a solid frame section that houses the steering wheel, pedals, and driver seat. In the event of a serious frontal impact, this section can slide back in its entirety and thus increase the driver’s space by several crucial centimeters.

Crash test “After completing our work with Group Research, we not only tested the effectiveness of the Front Collision Guard system using computer simulations; we also put it through its paces and validated its performance in several crash tests under real-life conditions,” Steinmetz reports. Between the fall of 2005 and the spring of 2007, six buses equipped with the system underwent extreme crash tests in which they were driven into a rigid barrier (see photos on p. 20).

Measurements taken of the crash test dummy used as a “driver” demonstrated the effective protection Front Collision Guard offers. “The biomechanical values were significantly lower than the limits stipulated by Euro NCAP,” says Averbeck. “For example, head acceleration upon collision and the forces impacting the thorax and thighs of the driver and front passenger were much lower than the permissible values.” In addition, the frontal crash tests showed that the bus passenger compartment also offers a high level of safety, as the passenger acceleration values were substantially lower than the permissible limits.

Averbeck summarizes the new safety features in the Travego as follows: “The combination of Active Brake Assist and Front Collision Guard results in an unprecedented level of safety for the driver and passengers.”
“Active Brake Assist will help prevent rear-end collisions involving buses.”
Katrin Breitrück, project manager
“The bus driver can override the automatic emergency braking system at any time.”
Katrin Breitrück, project manager
“ABA illustrates the good cooperation between Research, Trucks, and Buses.”
Richard Averbeck, Managing Director of EvoBus

Safety 12-pack


1 Active Brake Assist (ABA)
Automatic emergency braking if the driver fails to react to warnings and the bus is in danger of colliding with a vehicle in front.
2 Front Collision Guard (FCG)
Optimized support structure absorbs impact energy. Rigid frame that includes the driver seat slides back in a frontal collision.
3 Electronic Stability Program (ESP)
Automatically intervenes in braking and engine operations. Stabilizes the bus in critical situations to the extent physically possible.
4 Electronic Braking System (EBS)
Electronically controlled braking unit networked with a CAN data bus. Increases braking power, shortens braking distance.
5 Proximity control system
Radar system measures the distance to vehicles in front. Brakes automatically until a predefined distance has been reached.
6 Lane Assistant
A camera mounted behind the windshield determines if the bus is departing from the lane. Warns drivers by vibrating their seats.
7 Continuous braking limiter
Utilizes a retarder to brake the bus if it exceeds the speed limit of 100 km/h.
8 Pitch/roll control
Adjusts shock absorbers in line with payload and driving conditions. Provides for better safety when braking and cornering.
9 Litronic headlights and cornering lights
Xenon lamps with optimized reflectors provide better illumination of the road, making it easier to see the roadside and cross streets.
10 Rain/light sensor
Activates windshield wipers and adjusts them in line with rain intensity. Automatically turns on dipped headlights if needed.
11 Backup camera
Enables the driver to see everything happening behind the bus.
Exterior mirror with backup pilot
12 LEDs integrated into the mirrors indicate the distance to obstacles in the rear, thereby enabling safe reverse maneuvering.

let’s talk 90 sec. with…

Richard Averbeck

Richard Averbeck began his career at Daimler as a management trainee in 1982, initially working at the company’s Stuttgart headquarters and the Mannheim bus plant. In 1993, he was appointed Managing Director of Finance/Controlling at Mercedes-Benz Buses Mexico. Averbeck, who has a Master’s degree in Engineering Management, then transferred to EvoBus in 1995, where he has been Managing Director of Global Bus Development since 2001.

Why are buses considered to be the safest means of transport? This assessment is valid in terms of person-kilometers traveled. Statistically, trains and planes also do well, although rare but severe accidents negatively affect the statistics. Two things make buses so safe: The first has to do with their large number of technical safety systems, which intervene in dangerous situations. The second is that unlike car drivers, who also have the same systems, most bus drivers are well trained and drive responsibly, thus avoiding dangerous situations.

What role do assistance systems play in the lower number of bus accidents? Daimler Buses focuses on ensuring that it never even gets to the point where a critical situation can arise. If such a situation occurs, however, assistance systems can do a lot to help prevent an accident - either passively by providing drivers with appropriate warnings or actively through automatic intervention.

What contribution do passive safety features make here? Both active and passive safety systems are important, and this is shown clearly by Active Brake Assist (ABA) and the passive system Front Collision Guard (FCG). The two systems cover the final stages before and during an accident, which is why they’re both important elements of our safety strategy.

A lot depends on the bus driver - so how are drivers supported? Human beings - even well trained bus drivers - are the weak link in the chain. We therefore make keeping drivers alert and comfortable a top priority. We accomplish this with ergonomic driver areas and information systems, as well as through passive assistance systems that issue alarms, like the Lane Assistant. If drivers don’t react to these passive systems, active assistance systems like EBS, proximity control, and ABA then automatically come into play.

How does Daimler Buses plan to maintain its leadership in the area of safety in the future? For one thing, we’re continually working on new safety systems that focus on the driver. One such system recognizes signs of driver fatigue at an early stage and provides an appropriate warning. We are also continually refining and improving our existing assistance systems. Here, the overriding goal we are pursuing is to achieve the “Vision of Accident-Free Driving.”

Bus insurance

Assistance System Discounts

Both passive safety systems like Front Collision Guard and active systems like Active Brake Assist (ABA) help save lives. ABA, for example, helps prevent rear-end collisions, which is why its developers expect to see a lower number of bus accidents resulting in fatalities and severe injuries in the future.
This will also result in lower operating costs, as buses equipped with the ABA system receive a five percent discount on the liability and fully comprehensive insurance policies offered by Mercedes-Benz Bank.

Bus operators whose vehicles are equipped with the Lane Assistant and the proximity control system receive the same discount. If all three assistance systems are installed in the vehicle, the discount amounts to ten percent.

It also pays to order proximity control as an option for another reason - anyone who does so will be given Brake Assist (BA) as an added extra for free.

Emergency braking system

Four Awards for ABA

ABA, which recently celebrated its world bus premiere in the Travego coach, has been proving its worth in the heavy-duty Actros truck since 2006 and has won four awards:
In 2007, Germany’s ADAC automobile association presented a “Yellow Angel” - one of Europe’s most coveted automotive awards - to ABA in the category of Innovation.
Belgian automotive journalists also honored the system at the 2007 Brussels Motor Show with the Safety Award, which is presented every two years to the automaker that has done the most to promote the cause of safety.

In 2007, ABA also received the European Commercial Vehicle Safety Award presented by the technical appraisal organization DEKRA, the German Traffic Safety Council (DVR), and the European Association for Accident Research and Analysis (EVU).
Under the name “Protektor,” the emergency braking system made it to the final round of the competition for the German Future Prize back in 2002.


HTR Online contains further information on Active Brake Assist and bus safety in general.
Animated depictions of ABA and Front Collision Guard
The human factor: Mercedes-Benz training for bus drivers
Safety through ergonomics: The driver’s workstation


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