Initiative details
From July 2024, stricter EU regulations (GSR II) will come into force for all newly registered trucks and buses. Most of these also apply to city buses. An important exception in GSR II is the emergency braking assistance system, which is not mandatory for buses with unprotected standing and seated passengers. For this type of city bus, ZF has developed a brake assistance system that reduces the consequences of a possible collision. It contains a braking cascade which starts with a first braking pulse which enables sufficient reaction time for standing passengers in order to prepare for the actual braking maneuver, for example by holding and/or a compensating step. The deceleration is then raised to a conservatively selected value which is still controllable for standing passengers and at the same time represents a satisfactory balance with the desired reduction in the vehicle speed and the reduction in the severity of the collision. The Collision Mitigation System (CMS) protects both passengers and vulnerable road users and enables a safer urban public transport.
Initiative date
Who was/is your target audience?
Policy makers
Public authorities
Fleet operators
Car drivers – professional
Public transport
Cyclists
Micromobility riders
Powered two wheeler riders (excluding micromobility)
Pedestrians
Topic
Improve vehicles and infrastructure
Organisation details
ZF Group
Enterprise
Germany
Friedrichshafen
Contact name
Louise Forquenot de La Fortelle
Telephone number
+32 2 263 0861
louise.delafortelle@zf.com
Website link
Project activities
If you work together with external partners, list the most important partners and briefly describe their role.
The sensor set for the city bus brake assistant is based on ZF’s OnGuardMAX system and consists of a radar, a camera, and a data processing module. The data processing module is built on a powerful processor from Mobileye.
Real operational data from city buses were analyzed based on the number and intensity of braking events performed by the drivers.
Real operational data from city buses were analyzed based on the number and intensity of braking events performed by the drivers.
Please describe the project activities you carried/are carrying out and the time period over which these were implemented.
In recent years, ZF has extensively explored a middle ground that balances the conflicting goals of protecting passengers and drivers on one hand and safeguarding other road users on the other. On one side, real operational data from city buses were analyzed based on the number and intensity of braking events performed by the drivers. On the other side, a series of experiments were conducted with standing test subjects. Figure 1 attached schematically illustrates the braking cascade, where parameters were systematically varied. The goal of these experiments was to assess the manageability of brake interventions by passengers subjectively. In Figure 1, three stages of the braking cascade are visible. Each stage is characterized by the value of deceleration (a₁, a₂, a₃) in m/s², the so-called jerk (expressed in m/s³), and the duration for which the deceleration is sustained (D₁, D₂). The jerk describes the gradient of deceleration. The results show that the initial braking impulse (a₁, Jerk 1) as well as the maximum values for deceleration and jerk (a₂, Jerk 1) throughout the braking maneuver significantly influence the subjective assessment of controllability. However, the third braking (a₃, Jerk 3) did not improve overall controllability. Consequently, it was decided to raise the deceleration after the first braking impulse and a short holding phase, followed by a small jerk, directly to the maximum value represented by the dashed line in Figure 1. From this, a braking profile with a good balance between controllability for standing passengers and collision mitigation was derived.
For the initial braking, a deceleration of a₁ = -1.5 m/s² with a duration of D₁ = 0.5 s proved suitable, providing standing passengers with sufficient time for orientation and a step if needed. A longer braking duration would negatively impact collision avoidance without significantly improving controllability for passengers. For the main braking phase, a maximum deceleration of a₂ = -2.5 m/s² with a maximum jerk 2 of -16 m/s³ was found to be suitable. Higher jerk values significantly worsened controllability. As a result, a braking profile for city buses was developed, where the system initially brakes abruptly with a strong gradient but limited intensity. This provides standing passengers with an impulse to hold on and stabilize themselves if necessary before a stronger partial braking occurs. Through passenger “conditioning,” the maximum deceleration can be higher.
With this braking profile, a high speed reduction was achieved while maintaining acceptable controllability for standing passengers. The driver always has the option to override the system and request maximum braking force.
For the initial braking, a deceleration of a₁ = -1.5 m/s² with a duration of D₁ = 0.5 s proved suitable, providing standing passengers with sufficient time for orientation and a step if needed. A longer braking duration would negatively impact collision avoidance without significantly improving controllability for passengers. For the main braking phase, a maximum deceleration of a₂ = -2.5 m/s² with a maximum jerk 2 of -16 m/s³ was found to be suitable. Higher jerk values significantly worsened controllability. As a result, a braking profile for city buses was developed, where the system initially brakes abruptly with a strong gradient but limited intensity. This provides standing passengers with an impulse to hold on and stabilize themselves if necessary before a stronger partial braking occurs. Through passenger “conditioning,” the maximum deceleration can be higher.
With this braking profile, a high speed reduction was achieved while maintaining acceptable controllability for standing passengers. The driver always has the option to override the system and request maximum braking force.
In terms of implementation, what worked well and what challenges did you need to overcome?
In general there are 3 major challenges arising with applying an AEBS or CMS to city buses:
1) Safety related: Considering standing passengers in buses, a new set of hazards has to be considered in the system design – which is related to both, functional safety and safety of the intended functionality. It is about severity and controllability rating for standing passengers in case of unintended emergency braking interventions which can’t be avoided with given technology.
2) Urban Environment: Coming from heavy duty vehicles which are mainly operated on highways and rural roads, the CMS/AEBS for city buses is now permanently exposed to a dense and crowded environment.
3) Crossing pedestrians (challenge also known from passenger car) with unclear intention whether to continue the movement or suddenly stop at the road border are challenging for the system.
1) Safety related: Considering standing passengers in buses, a new set of hazards has to be considered in the system design – which is related to both, functional safety and safety of the intended functionality. It is about severity and controllability rating for standing passengers in case of unintended emergency braking interventions which can’t be avoided with given technology.
2) Urban Environment: Coming from heavy duty vehicles which are mainly operated on highways and rural roads, the CMS/AEBS for city buses is now permanently exposed to a dense and crowded environment.
3) Crossing pedestrians (challenge also known from passenger car) with unclear intention whether to continue the movement or suddenly stop at the road border are challenging for the system.
Evaluation
Please summarise how you have evaluated the initiative’s impact (e.g. social media reach, survey, feedback forms, statistics).
ZF has engaged directly with the whole urban bus ecosystem (OEMs, fleets and municipalities) to get their feedbacks and experiences of emergency braking situations.
First our expert Dr. Richard Matthaei in his role as PE Technology Expert ADAS presented the results of their research on braking profiles at the ATZ conference in Wiesbaden.
Our ADAS experts also spent two full days with a team from VDL, a large coach and city-bus manufacturer, at our test track in Jeversen, Germany. A joint team of volunteers from VDL and ZF equipped with safety harness, helmet, as well as a questionnaire went on a VDL city-bus to safely test, document, and evaluate the braking experience from passenger perspective with increasing vehicle speeds and brake intensities to investigate how much deceleration intensity is acceptable at which speed for the passengers.
Also, ZF showcases the Collision Mitigation System in various fairs to engage with the experts of the sector, as for example at the Busworld 2023 in Brussels, and at the UITP Global Public Transport Summit 2023 in Barcelona.
First our expert Dr. Richard Matthaei in his role as PE Technology Expert ADAS presented the results of their research on braking profiles at the ATZ conference in Wiesbaden.
Our ADAS experts also spent two full days with a team from VDL, a large coach and city-bus manufacturer, at our test track in Jeversen, Germany. A joint team of volunteers from VDL and ZF equipped with safety harness, helmet, as well as a questionnaire went on a VDL city-bus to safely test, document, and evaluate the braking experience from passenger perspective with increasing vehicle speeds and brake intensities to investigate how much deceleration intensity is acceptable at which speed for the passengers.
Also, ZF showcases the Collision Mitigation System in various fairs to engage with the experts of the sector, as for example at the Busworld 2023 in Brussels, and at the UITP Global Public Transport Summit 2023 in Barcelona.
What has been the effect of the activities?
Our direct engagement with OEMs and fleets allows us to better understand real-world scenarios and challenges related to emergency braking.
By assessing braking experiences directly from the passenger viewpoint, they aimed to strike a balance between collision mitigation and passenger well-being.
By assessing braking experiences directly from the passenger viewpoint, they aimed to strike a balance between collision mitigation and passenger well-being.
Please briefly explain why your initiative is a good example of improving road safety.
The Collision Mitigation System (CMS) significantly enhances road safety for bus passenger as well as vulnerable road users. The system also streamlines public transport deployment in cities. In a context where micromobility is considerably growing in Europe and urban public transport is an essential component of a transition towards greener mobility, such road safety benefits will only gain in relevance in the future.
How have you shared information about your project and its results?
https://www.zf.com/products/en/cv/products_69633.html#contact_divider658792
https://link.springer.com/chapter/10.1007/978-3-658-34752-9_4
https://link.springer.com/chapter/10.1007/978-3-658-34752-9_4