Author: Dr Ciaran Simms, assistant professor in the School of Engineering at Trinity College Dublin and principal investigator in the Trinity Centre for Bioengineering Pedestrian traffic accidents are complex; protecting pedestrians requires a combination of injury biomechanics research, road engineering, vehicle design, legislation, and accident-avoidance technology. Separating pedestrians from fast-moving, motorised vehicles is preferable, and pre-crash sensing methods and brake-assist technology can now reduce the occurrence and severity of pedestrian accidents. However, vehicle-pedestrian collisions will remain a significant problem for the foreseeable future in all countries. This article provides an overview of the main factors influencing pedestrian injuries in the event of a collision, and gives a summary of vehicle-based pedestrian protection systems and areas of ongoing pedestrian injury biomechanics research. The correlation between vehicle impact speed and pedestrian injury severity is well documented, and recent data shows a steep increase in fatality risk above about 40 kilometres per hour (km/h) . In most cases, after being impacted by a vehicle, the pedestrian strikes the ground and significant injuries can be attributed to both the vehicle and the ground impacts , especially for child pedestrians. We have known since the 1970s that vehicle design has a significant effect on the severity of pedestrian injuries during vehicle impact , and there is a strong relationship between vehicle class (cars, trucks, SUVs, etc) and pedestrian injury and fatality risk. [login type="readmore"] ODDS RATIO FOR PEDESTRIAN FATALITIES A recent analysis of accident data found that the odds ratio for pedestrian fatalities from light trucks and vans (LTVs) impacts compared to cars is close to 1.5 . This is because LTVs are high-fronted vehicles and cause significant midbody injuries (see Figure 1).
Figure 1: Distribution of serious and fatal pedestrian injuries as a function of vehicle type: cars versus LTVs, adapted from . The movement of pedestrians during impact helps in understanding the resulting injuries caused and how to reduce them. The dominant factors are the shape and speed of the vehicle and the height, walking phase and speed of the pedestrian with respect to the vehicle. In most accidents, the pedestrian was crossing the road when struck from the side by a vehicle, and pedestrian ‘wrap projection’ occurs when the main impact force is applied below the person’s centre of gravity – for example, when an adult is struck by a passenger car. This is the most common pedestrian collision configuration and the first impact with the legs is generally followed by a further contact where the head is struck by the bonnet or windscreen area of the vehicle (see Figure 2), which shows a 40km/h staged pedestrian impact with a cadaver . 0ms 40ms 80ms 140ms Figure 2: Pedestrian wrap projection for a cadaver test at 40 km/h, adapted from . In contrast, ‘forward projection’ motion occurs when the pedestrian centre of gravity lies within the primary impact zone. This happens when the pedestrian stature is small compared to the vehicle bonnet leading-edge height, and there is much less body rotation during the impact phase. These kind of impacts lead to severe midbody injuries (see Figure 1). VEHICLE DESIGN ISSUES The two main vehicle design issues for pedestrian protection are shape and stiffness, and mass only has a secondary effect. Vehicle stiffness is a fundamental parameter for pedestrian injury and shape is important because it determines the impact locations on the body. Furthermore, the shape influences the energy associated with each impact, while stiffness determines the corresponding force. Therefore shape and stiffness combine to determine injury likelihood for a pedestrian when struck by a vehicle at a particular speed. The bumper height is important in the risk of damage in the knee. A bumper striking close to, or above, the knee is considerably more damaging than a bumper striking below the knee . A significant reduction in knee injuries can be achieved by the provision of a secondary bumper close to the ground which supports the lower leg, and these are now commonplace. The ideal vehicle bumper for pedestrian protection allows sufficient compliance to cushion the impact while limiting knee lateral bending. However, a challenge is posed by bumper regulations which are designed to minimise vehicle repair costs. These regulations generally require the bumper structure to absorb considerable energy, thus limiting decreases in bumper stiffness which would help pedestrians. The bonnet leading edge is also a source of injuries, especially for high-fronted vehicles (see Figure 1) and airbags have been proposed to address this (see Figure 3), though these are not yet available in production vehicles.
Figure 3: SUV bonnet leading edge airbag, adapted from . Deformation space is needed when the head strikes the bonnet or windscreen (see Figure 2) and several designs have been proposed to achieve this through a lifting bonnet in the event of a pedestrian impact (see Figure 4). Staged tests with crash test dummies have indicated their ability to reduced head injury risks to acceptable levels in head bonnet impacts for a 40 km/h impact . Figure 4. Lifting bonnet and A-Pillar airbag for pedestrian head protection, adapted from . Active safety or impact prevention strategies have made a lot of progress in the last decade. The ‘brake assist’ system in the brake pedal senses the braking motion of the driver and optimises braking performance and these were mandated in new vehicles in Europe in 2008. In future, these primary safety measures (brake assist) and secondary safety measures (lifting bonnet, external airbags) will be combined into integrated systems, but this is not yet standard in production vehicles. PUBLIC INTEREST There is now significant public interest in the assessment and regulation of vehicle design for pedestrian safety. In Europe we have an EU directive dedicated to this topic and the independent New Car Assessment Programme (EuroNCAP), in which pedestrian safety is assessed via impactors representing the different body regions. A recent study comparing EuroNCAP pedestrian scores with accident outcomes in Sweden showed a significant reduction of injury severity for cars with better pedestrian scoring, indicating some success with the regulatory approach . Pedestrian injury biomechanics research combines physical testing of tissues and limbs and whole-body testing using cadavers and crash test dummies and computational modelling to improve our understanding of the impact processes. These processes are highly variable, and depend on subject specific biomechanics as well as the initial gait stance of the pedestrian and the detailed design of the vehicle. However, the fruits of this research have been significant, with large decreases in the fatality risks over the last few decades when pedestrians are struck by vehicles. These topics continue to be a research focus for the Injury Biomechanics research group in the Centre for Bioengineering at Trinity College Dublin [e.g. 12-15]. In summary, pedestrian impacts are highly variable but protection can be achieved through a combination of vehicle shape configuration, structural stiffness reductions and active safety devices such as autonomous braking, lifting bonnets and airbags. However, there is a large variability in the aggressivity of the current vehicle fleet for pedestrians, and we still do not know which design is optimal for pedestrian protection . Dr Ciaran Simms, FTCD, MIEI is an assistant professor in the School of Engineering at Trinity College Dublin. He is also a principal investigator in the Trinity Centre for Bioengineering. Prior to working in TCD, he held research engineer positions with TNO Automotive in Delft and Detroit and with Denis Wood Associates in Dublin. His teaching and research interests are in injury biomechanics, soft-tissue mechanics and medical-device design. He is contactable at firstname.lastname@example.org. References 1. Rosen, Erik and Ulrich Sander, Pedestrian fatality risk as a function of car impact speed. Accident Analysis & Prevention, 2009. 41(3): p. 536-542. 2. Foret-Bruno, JY., G. Faverjon, and JY. Le Coz. Injury pattern of pedestrians hit by cars of recent design. in Experimental Safety Vehicles Conference. 1998. 3. Simms, CK. and DP. Wood, Pedestrian and Cyclist Impact - a Biomechanical Perspective. 2009: Springer. 4. Desapriya, E., S. Subzwari, D. Sasges, A. Basic, A. Alidina, K. Turcotte, and Pike. I., Do Light Truck Vehicles (LTV) Impose Greater Risk of Pedestrian Injury Than Passenger Cars? A Meta-analysis and Systematic Review. Traffic Injury Prevention, 2010. 11(1): p. 48-56. 5. Longhitano, D., J. Ivarsson, B. Henary, and J. Crandall. Torso injury trends for pedestrians struck by cars and LTVs. in Experimental Safety Vehicles Conference. 2005. 6. Kerrigan, J., D B. Murphy, DC. Drinkwater, CY. Kam, D. Bose, and J. Crandall. Kinematic corridors for PMHS tested in full-scale pedestrian impact tests. in Experimental Safety Vehicles Conference. 2005. 7. Ishikawa, H., K. Yamazaki, and A. Sasaki. Current situation of pedestrian accidents and research into pedestrian protection in Japan. in Experimental Safety Vehicles Conference. 1991: p. 281-291. 8. Fredriksson, R., E. Flink, O. Bostrom, and K. Backman. Injury mitigation in SUV-to-pedestrian impacts. in Experimental Safety Vehicles Conference. 2007. 9. Fredriksson, R., Y. Haland, and J. Yang. Evaluation of a new pedestrian head injury protection system for the sensor in the bumper and lifting of the bonnets rear part. in Experimental Safety Vehicles Conference. 2001. 10. Kuehn, M., R. Froeming, and V. Schindler. Assessment of vehicle related pedestrian safety. in Experimental Safety Vehicles Conference. 2005. 11. Strandroth, J., M. Rizzi, S. Sternlund, A. Lie, and C. Tingvall, The Correlation Between Pedestrian Injury Severity in Real-Life Crashes and Euro NCAP Pedestrian Test Results. Traffic Injury Prevention, 2011. 12(6): p. 604-613. 12. Simms, CK. and D. Wood, Effects of pre-impact pedestrian position and motion on kinematics and injuries from vehicle and ground contact. International Journal of Crashworthiness, 2006. 11(4): p. 345-356. 13. Elliott, JR., CK. Simms, and DP. Wood, Pedestrian head translation, rotation and impact velocity: The influence of vehicle speed, pedestrian speed and pedestrian gait. Accident Analysis & Prevention, 2012(45): p. 342-353. 14. Simms, C. and D. Wood, Pedestrian risk from cars and sport utility vehicles - a comparative analytical study. IMechE Journal of Automobile Engineering, 2006. 220: p. 1085-1100. 15. Lyons, M. and CK. Simms. Predicting the Influence of Windscreen Design on Pedestrian Head Injuries. in proceeding of IRCOBI Conference. 2012. Dublin: p. IRC-12-77.