Pedestrian safety through vehicle design
Road vehicles designed to be slower and lighter are generally safer for pedestrians. A quarter of a million pedestrians lost their lives on the world’s roads in 2021. Despite the magnitude of the problem, most attempts at increasing pedestrian safety had historically focused solely on education and traffic regulation. Since the 1970s, 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, or reduce the likelihood of a car-pedestrian crash in the first place.
These involve redesigning the bumper, hood, windshield and pillar to be energy absorbing without compromising the structural integrity of the car. With the advent of ADAS since 2005, new pedestrian detection and crash avoidance and mitigation systems offer improvements through active rather than passive protection systems. For example, omniview technology allows a driver to see what is around the vehicle before moving. More regions are requiring heavy vehicles to have direct vision of pedestrians, rather than relying on mirrors.
Anatomy of a pedestrian crash
Many 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. The pedestrian is impacted twice, first by the car and then by the ground, but most of the fatal injuries occur due to interaction with the car. Vehicle designers usually focus 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 pedestrian deaths occur 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 are the most common cause of disabilities. A Frontal Protection System is a device fitted to the front end of a vehicle to protect both pedestrians and cyclists in the event of a front-end collision. Car design has been shown to have a large impact on the scope and severity of pedestrian injury in car crashes.Volvo has created a pedestrian and cycle recognition ADAS with automatic braking designed to reduce pedestrian collisions. Some car models also have 'dooring' technology, to prevent car doors being opened onto oncoming cyclists. With pedestrian injuries and fatalities increasing dramatically in the US in 2017, due to factors including larger cars and increasingly distracted driving with entertainment and communication systems in cars, pedestrian safety driver support systems may become widespread.
Protecting the head
The hood of most vehicles is usually fabricated from sheet metal, which is a compliant energy absorbing structure which 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 death. 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. Some models, like the Citroën C6 and Jaguar XK feature a novel pop-up bonnet design, which adds 6.5 cm extra clearance over the engine block if the bumper senses a hit. In 2012 and 2015, the Volvo V40 and the Land Rover Discovery Sport have an under-the hood airbag designed to operate if the hood senses a hit. The airbag is also designed to cover the windshield pillars to help protect the pedestrian's head. However, these pedestrian airbags have not been taken up widely.