Pedestrian safety through vehicle design

Almost two-thirds of the 1.2 million people killed in road traffic crashes worldwide are pedestrians. Despite the magnitude of the problem, most attempts at reducing pedestrian deaths have focused solely on 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 pedestrians in the event of a car-pedestrian crash. These involve redesigning the bumper, hood (bonnet), and the windshield and pillar 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 speed of the car and then continues 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 the pedestrian, the leading edge of the hood hits the upper thigh or pelvis, and the head and upper torso are struck by the top surface of the hood and/or windshield.

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 usually 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. A Frontal Protection System (FPS) is a device fitted to the front end of a vehicle to protect both pedestrians and cyclists who are involved in a front end collision with a vehicle.

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 insufficient clearance between the hood and the stiff-underlying engine components. A gap of approximately 10 cm is usually enough to allow the pedestrian’s head to have a controlled deceleration and a significantly reduced risk of dea. Creating room under the hood is not always easy because usually there are other design constraints, such as aerodynamics and styling. In some regions of the hood it can be impossible. These include along 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 by developing more ambitious solutions such as airbags that are activated during the crash and cover the stiff regions of the hood. The 2006 year model of Citroën C6 and Jaguar XK feature a novel pop-up bonnet design, which adds 12cm (5") extra clearance over the engine block if the bumper senses a hit.

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 tibula/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 when cars have lower bumpers, the thigh and leg rotate together causing the knee to bend less and thus reducing 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.