Pedestrian safety through vehicle design
Almost two-thirds of the 1.2 million people killed in road traffic crashes worldwide are pedestrians [1]. Despite the magnitude of the problem, most attempts at reducing pedestrian deaths have focused solely on isolating pedestrians from cars, education and traffic regulation. However, in recent years crash engineers have begun to use design principles that have proved successful in protecting car occupants to develop vehicle design concepts that reduce the likelihood of injuries to the pedestrian in the event of a car-pedestrian crash. These involve redesigning the bumper, hood (bonnet), and the windshield to be energy absorbing (softer) without compromising the structural integrity of the car.
Anatomy of a Pedestrian Crash
Most pedestrian crashes involve a forward moving car. In such a crash, a standing or walking pedestrian is struck and accelerated to the same speed as that of the car and then thrown forward as the car brakes to a halt. Although the pedestrian is impacted twice, first by the car and then by the ground, most of the fatal injuries occur due to the interaction with the car. Thus vehicle designers usually focus their attention on understanding the car-pedestrian interaction which is characterized by the following sequence of events: the vehicle bumper first contacts the lower limbs of a pedestrian, the leading edge of the hood hits the upper thigh or pelvis region, the head and upper torso is struck by the top surface of the hood and/or windshield, and finally the pedestrian is thrown forward and strikes the ground as the car brakes to a halt.
Reducing Pedestrian Injuries
Most pedestrians die due to the traumatic brain injury resulting from the hard impact of the head against the stiff hood or windshield. In addition, although non-fatal, injuries to the lower limb (usually to the knee joint and long bones), are the most common cause of disability due to pedestrian crashes.
Protecting the Head
The hood of most vehicles is usually fabricated from sheet metal, which is a compliant energy absorbing structure and thus poses a comparatively small threat. Most serious head injuries occur when there is not sufficient clearance between the hood and the stiff-underlying engine components. A clearance of approximately 10 cm is usually sufficient to allow the pedestrian’s head to have a controlled deceleration and a significantly reduced risk of death. Creating room under the hood is not always easy because usually there are other design constraints, such as aerodynamics and styling. In some region of the hood it can be impossible. These include the edges on which the hood is mounted and the cowl, where the hood meets the windshield. Engineers have attempted to overcome this problem by using deformable mounts, and more ambitious devices such as airbags that are activated during the crash and cover the stiff regions of the hood [2].
Protecting the Limbs
Most limb injuries occur due to a direct blow from the bumper and the leading edge of the hood. This leads to contact fractures of the femur and the tibia/fibula and damage to the knee ligaments due to bending of the joint. Thus attempts at reducing these injuries involve reducing the peak contact forces (by making the bumper softer and increasing the contact area) and by limiting the amount of knee bending by modifying the geometry of the front end of the car. Computer simulations and experiments with cadavers show that lower bumpers permit the thigh and leg to rotate together causing the knee to bend less and thus reduce the likelihood of ligament injuries. Deeper bumper profiles and structures under the bumper (such as the air dam) can also assist in limiting the rotation of the leg.
References
- [1] http://www.worldbank.org/transport/roads/safety.htm
- [2] http://www.autoliv.com/appl_alv/Autoliv.nsf/pages/pedestrian_protection