Pedestrian safety through vehicle design

More than 270 000 pedestrians lose their lives on the world’s roads each year accounting for 22% of the total 1.24 million road traffic deaths . Despite the magnitude of the problem, most attempts at reducing pedestrian deaths had historically focused solely on education and traffic regulation. Since the 1990s, 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

The sequence of events in a car-pedestrian crash

Most pedestrian crashes involve a forward moving car (as opposed to buses and other vehicles with a vertical hood/bonnet). 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. The 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 pedestrian deaths occur due to the traumatic brain injury resulting from the hard impact of the head against the stiff hood or windshield^[2]. 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. Car design has been shown to have a large impact on the scope and severity of pedestrian injury in car crashes.^[3]

While the lower limb is the most commonly injured body region, most pedestrian fatalities are due to head injuries^[3].

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 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 (2.5", C6) 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 which operates if the hood senses a hit. The airbag also covers the windshield pillars to provide better protection in all areas struck by the pedestrian's head.

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 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.

Trams

A protective shield in front of a Helsinki tram, ca 1975. When a person hits the detector (yellow arrow), a shield (red arrow) launches to prevent the person from going under the wheels.

An early example can be found on trams in the form of a lifeguard which prevents pedestrians from being caught between the wheels of the leading bogie should they be hit. When a pedestrian hit the lifeguard a scoop/grille would be automatically lowered in front of the vehicle. This protects the tram against derailment as well as reducing the likelihood of the pedestrian being killed.

References

^ http://www.who.int/mediacentre/news/notes/2013/make_walking_safe_20130502/en/
^ ^a Hamer, M. (27 August 2005). "Stopping the slaughter of innocent pedestrians". New Scientist (2514).
^3 Jain, SL. (February 2004). ""Dangerous Instrumentality": The Bystander as Subject in Automobility". Cultural Anthropology. 91 (1).
^ ^a Crandall, JR, Bhalla, K, and Madeley, NJ (11 May 2002). "Designing road vehicles for pedestrian protection". British Medical Journal. 324. doi:10.1136/bmj.324.7346.1145.
^ http://www.autoliv.com/appl_alv/Autoliv.nsf/pages/pedestrian_protection
^ Bunketorp O, Romans B, Hansson T, Aldman B, Thorngren L, Eppingen RH. "Experimental Study of a Compliant Bumper System". Proceedings of the 27th Stapp Car Crash Conference. SAE Paper No. 831623

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