C OVER STORY S AFE T Y
IN-CAR SAFETY SYSTEMS FOR PEDESTRIAN AND CYCLIST SAFETY Protecting vulnerable road users such as pedestrians and cyclists requires further innovations. In cooperation with Autoliv, TNO in the Netherlands has developed the prototype of a new windscreen airbag and an extension to the emergency braking function.
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AUTHORS
STEFANIE DE HAIR-BUIJSSEN is SaveCap Project Manager working within the Integrated Vehicle Safety Department of TNO in Helmond (Netherlands).
CARMEN RODARIUS is a VRU Expert at the Integrated Vehicle Safety Department of TNO in Helmond (Netherlands).
MARGRIET VAN SCHIJNDEL-DE NOOIJ is Project Manager in the Integrated Vehicle Safety Department of TNO in Helmond (Netherlands).
IMPROVE SAFET Y
Over the past few decades, much has been done to increase road safety and to reduce the number of traffic fatalities and serious injuries. Although overall road casualty numbers have decreased significantly, the number of pedestrian and cyclist casualties remains constant or has dropped only slightly. The number of cyclists is increasing not only in Germany and the Netherlands, but also in other countries. This is supported, for example, by providing public bicycles in cities and by the introduction of e-bikes. Thus, it is inevitable that dedicated attention is needed to protect vulnerable road users (VRUs). As cars are a very common crash partner for VRUs, the SaveCap project [1] paid special attention to two potential measures on cars to help mitigate or even avoid crashes with VRUs. The measures investigated were a VRU airbag and an automatic emergency braking system for VRUs. Despite general traffic safety measures, the number of casualties among
VRUs is not decreasing, and the size of the group is expected to increase in the near future, ❶. Dedicated protection is therefore vital. The SaveCap project (2008 to 2012) aimed at providing tools for increased VRU safety through the development of proof of concept systems to protect both cyclists and pedestrians in collisions with passenger cars. ACCIDENT ANALYSIS
The kinematics of pedestrians hit by a car have been studied by various people over the past few decades. Information on carto-cyclist accidents, however, is rare. A study of the literature offered an insight into the VRU injury mechanisms, the body regions injured and the parts that caused injuries. For VRUs, head and chest injuries are the most dominant and most severe [2]. Fatal cases from Gidas (German In-Depth Accident Study) show that 80 % of the VRUs killed sustained severe head and/or chest injuries (AIS3+) and 65 % sustained severe injuries to both body parts. In the severely injured
Traffic fatalities the Netherlands 1400 1200 1000 800
RIKARD FREDRIKSSON is VRU Researcher and Manager of the Biomechanics & Restraints Department at Autoliv Research in Vårgåda (Sweden).
Total Cyclists
600
Pedestrians 400 200 0 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
Traffic fatalities Germany 8000 7000 6000 5000 Total
4000
Cyclists
3000
Pedestrians
2000 1000
❶ Traffic fatality figures (total, cyclists, pedestrian) in Germany and the Netherlands 10I2013
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0 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
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C OVER STORY S AFE T Y
VRU cases, leg injuries are most dominant, as 60 % received severe injuries to the lower extremities. The majority of the severe head and chest injuries can be attributed to impacts with either the windscreen area or the bonnet of the car. With regard to the accident configurations, both types of VRU suffer most from a side impact and secondly from a rear impact [3]. SIMULATIONS AND RECONSTRUCTIONS
Over 20,000 simulations, ❷, in combination with real world accident data gave a detailed insight into accidents between passenger cars and VRUs with regard to VRU kinematics, the impact location of different body parts and the severity of injuries. The simulation parameters included, for example, the vehicle type and shape, the body size of the VRU and the impact speed. There are some significant differences in kinematics between cyclists and pedestrians [4]. Pedestrians have direct contact with the ground, whereas for the cyclist the bicycle is in between. Furthermore, the centre of gravity will usually be lower when walking. Whereas the legs of the pedestrian wrap around the bumper of the car during the impact, the cyclist is much more free to slide over the bonnet of the car. Hence, the pedestrian’s head will hit the car (bonnet or windscreen) lower than the cyclist’s head (windscreen and in some cases the roof). Therefore, not all countermeasures designed to protect pedestrians are automatically also effective for cyclist protec-
❷ Madymo simulation of cyclist impact by car
tion. Pre-crash experiments and simulations gave an insight into the manoeuvrability of cyclists, which can also be used for the specification of parameters such as airbag service life and design. As a final step in the study on VRU kinematics and injury mechanisms, SaveCap conducted a set of crash tests. Testing was done with a headform and with a dummy, standing or riding a bike. Two countermeasures were investigated: VRU airbag and AEB VRU. Both use a sensor system that detects cyclists and pedestrians in front of the car. VRU AIRBAG
The SaveCap VRU airbag is a U-shaped airbag, ❸, that covers the most critical areas of the windscreen area, in other words stiff areas in which the majority of the severe head impacts of VRUs are caused. The SaveCap airbag system consists of a dual sensor system and a dual protection system. The pre-crash sensor is a 40° field-of-view stereo-vision camera. The contact sensor consists of several
accelerometers along the bumper width. The stereo-vision sensor detects VRUs in the vehicle path and the contact sensor detects the VRU impact to the bumper to distinguish them from other objects and to confirm the physical impact. Then, a firing signal is sent to the protection system, consisting of two hood lifters and an airbag. The hood is lifted to increase the distance to the engine block in order to mitigate upper body injuries and to allow the airbag to inflate and cover the lower windscreen and the A-pillars. The protection system can only be triggered while driving and is designed to give optimalprotection up to 45 km/h. The SaveCap airbag differs in two perspectives from airbags currently on the market. The SaveCap airbag is an outer airbag. It has a similar type of design to an occupant airbag, but the module has to withstand more extreme environmental conditions such as rain and snow. It protects both cyclists and pedestrians, covering a larger area than a pedestrian airbag, as cyclists impact higher on the windscreen. It is therefore larger in volume than a pedestrian-only airbag and it requires a dedicated sensor to detect both groups in time. The main challenges for an effective VRU airbag are the bag size, the service life and deployment. For airbag deployment, a large gas generator is needed. In order to offer protection for smaller and larger cyclists and pedestrians at both low and higher speeds, the airbag has to maintain the pressure for several 100 ms. The triggering of the outer airbag requires the integration of pre-crash and contact sensing. AEB VRU
❸ Prototype VRU airbag
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In the SaveCap system, a stereo-vision camera is used to detect VRUs and to calculate the time-to-collision (TTC).
❹ Prototype stereovision VRU detection sensor
When the collision is unavoidable and TTC is within a predefined value, autonomous emergency braking (AEB) is activated. This starts with a warning, followed, if needed, by either a low braking force initially and then by a high braking force, if there is no reaction from the driver, or immediately by a high level of braking. SENSOR SYSTEM
The sensor system must be completely reliable, triggering only when absolutely needed. The VRU’s ability to instantly stop, reverse or change direction is a challenge for sensor development. A sensor field test (SFT) was performed during the development process. Real-life data was collected by in-car prototype sensor systems, ❹. These systems recorded VRU critical events. The SFT run for a period of almost two years, using five test vehicles in two large Dutch cities. Data was gathered from approximately 60,000 km and 3000 h of driving time. The SFT offered insight into very typical near-impact scenarios between passenger cars and VRUs, ❺. It resulted in a set of typical scenarios to be covered by a sensor system, including the timing and distances between the potential impact partners. It offered insight into the actual and rather unpredictable behaviour and movement of VRUs when close to a passenger car. Due to the SFT, the false positive rate of the sensing system was reduced by 50 %, and came into a range for possible seriesproduction application. 10I2013
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❺ Example of footage obtained during SFT
The same pre-crash sensor can be used for cyclists and pedestrians as long as both VRUs’ characteristics and needs are taken into account in the development process. In critical situations, the cyclist is usually crossing close to junctions, while pedestrians more often appear behind parked cars, etc. The fact that cyclists have a higher lateral speed also leads to a higher proportion of hitting the corners or even the side for the first impact and still hitting the windscreen area with the head. Thus, extra accelerometers farther out on the corner of the car front are needed in addition to those for a pedestrian-only protection system. The VRU detection system for AEB and airbag can be the same, as they both trigger in the same critical situations. The performance criteria for AEB camera sensors are higher than for the airbag, as AEB does not have a second (contact) sensor. A very restricted number of false positives could be acceptable for near misses for AEB , while for the VRU airbag system, the number of false positives must be minimised towards zero.
VeHil facility at TNO and Autoliv’s Carson City outdoor facility. The objective of the VeHil tests was to develop a test method to evaluate the detection possibilities of VRU detection systems. In the VeHil tests, the scenarios can be predefined in detail, with the result that a production system can be challenged to and beyond its design limits. The purpose of the Carson City testing was to ensure that the False Negative rate is sufficiently low. The prototype sensor system for the SaveCap airbag and AEB was tested with a pedestrian dummy and dummy cars. Full-scale car to cyclist crash tests showed the overall injury mitigation potential of the two safety systems. A normal car was equipped with a proof of concept VRU airbag. The scenario configurations tested were a pedestrian/cyclist (Polar II on an standard Dutch men’s bike) impacted by a passenger car from the side and impacted from the rear. Three types of tests were carried out: reference sets (no countermeasures), VRU airbag tests and full braking test. The results of these tests are shown in ❻.
SYSTEM TEST
EFFECTIVENESS
To evaluate the collision avoidance and mitigation functionality of future production vehicles equipped with ADAS for VRU protection, internationally approved crash tests and test methods are required. Within SaveCap, initial steps towards this are being taken, based on accident analysis and reconstructions. Pre-crash tests used the indoor
In SaveCap, the potential effectiveness in terms of reduced injuries or fatalities has been estimated based on Dutch police data [5] and German Gidas data [6]. The VRU airbag can potentially reduce the number of cyclists and pedestrians killed or suffering serious head injuries in a collision with a passenger vehicle by 36 %, an AEB VRU
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HEAD INJURY CRITERIA HIC15
Cyclist side impact, contact point on far side, head on A-pillar 1
Reduced braking (33 kph, 25 deg pitch)
749
2
Full braking (26 kph, 50 deg pitch)
806
3
Airbag
627
4
Airbag
499
Cyclist side impact, contact point on vehicle center, head on lower windscreen 1
Airbag
Airbag
The SaveCap project was sponsored by the Dutch government and the Swedish government via Vinnova FFI. The project partners were TNO, Autoliv, the Dutch Cyclists’ Union and Centraal
258
Beheer Achmea. Our special thanks go to KPN for their role in the SFT as well as to Honda for
Cyclist rear impact, contact point on far side, head on A-pillar 2
THANKS
357
providing the Polar II dummy.
❻ Results of crash tests for cyclist-passenger car impact at 40 km/h
system by 38 to 46 % and the optimum solution, a combination of both systems, by 64 to 72 % [7]. CONCLUSIONS
SaveCap has succeeded in delivering a proof of concept VRU airbag and AEB VRU that offer protection to both cyclists and pedestrians. These were made public at a demo event in 2012. Beside this, the project delivered knowledge on the development of VRU protection measures, as well as their assessment and potential. The SaveCap project has shown that both cyclist and pedestrian
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accidents can be effectively avoided with the same countermeasures when these are carefully developed. Several systems for AEB are on the market, as is a first cyclist safety AEB system. For pedestrian protection, the first windscreen airbag is on the market. If there ist interest from the car buyers, this can be developed further and developed into ultimate VRU protection from a car perspective. The most challenging part of the VRU protection by AEB or airbag remains reliable VRU detection, as VRU behaviour is hard to predict and the user acceptance of false positives will be extremely low.
REFERENCES [1] www.savecap.org [2] Van Schijndel, M. et. al.: VRU paradise goes for the next safety level, ESV 2011 [3] Fredriksson, R. et al: Priorities for Bicyclist Protection in Car Impacts – a Real life Study of Severe Injuries and Car Sources, Ircobi 2012 & Fatal vehicle-to-byciclist crashes in Sweden – an in-depth study of injuries and vehicle sources [4] Rodarius, C. et al: Bicycle safety in bicycle to car accidents, TNO-033-HM-2008-00354, April 2008 [5] Dutch National statistics database Bron [6] German Indepth Database Gidas, www.gidas.org [7] Fredriksson, R. et. al.: Potential of protection systems for vulnerable road users, ICSC 2012 conference
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