Education and debate

Designing road vehicles for pedestrian protection

J R Crandall, director, K S Bhalla, research associate, N J Madeley, orthopaedic research fellow. 

Center for Applied Biomechanics, University of Virginia, 1011 Linden Avenue, Charlottesville, Virginia 22902 USA

Correspondence to: J R Crandall jrc2h{at}virginia.edu

Collisions between pedestrians and road vehicles present a major challenge for public health, trauma medicine, and traffic safety professionals. More than a third of the 1.2 million people killed and the 10 million injured annually in road traffic crashes worldwide are pedestrians.¹ Compared with injured vehicle occupants, pedestrians sustain more multisystem injuries, with concomitantly higher injury severity scores and mortality.² Although a disproportionately large number of these crashes occur in developing and transitional countries, pedestrian casualties also represent a huge societal cost in industrialised nations. In Britain pedestrian injuries are more than twice as likely to be fatal as injuries to vehicle occupants³ and result in an average cost to society of £57 400, nearly twice that of injuries to vehicle occupants.⁴

Despite the size of the pedestrian injury problem, research to reduce traffic related injuries has concentrated almost exclusively on increasing the survival rates for vehicle occupants. Most attempts made to reduce pedestrian injuries have focused solely on isolation techniques such as pedestrian bridges, public education, and traffic regulations and have not included changes to vehicle design. The lack of effort devoted to vehicle modifications for pedestrian safety has stemmed primarily from a societal view that the injury caused by a large, rigid vehicle hitting a small, fragile pedestrian cannot be significantly reduced by alterations to the vehicle structure. Crash engineers, however, have long been aware that the same principles of car safety design that have produced enormous benefits for vehicle occupants can be extended to provide a safer environment for pedestrians during impact with the exterior of a vehicle.

Fig 1.   Vehicle impact zones in pedestrian-vehicle crashes. Vehicle fronts are responsible for most pedestrian injuries. (Data from US Department of Transportation National Highway Traffic Safety Administration5 and refer to right hand drive vehicles)

Fig 2.   Sequence of events in a pedestrian-vehicle crash: after the vehicle bumper hits the lower limb, the pedestrian "wraps around" the car, causing the head to strike the windscreen or bonnet

In the past, vehicle design concepts for pedestrian safety have not coincided with the changes in public attitude, vehicle regulatory testing, and safety technology that are necessary to develop a holistic approach to the problem. However, current cultural, scientific, and legislative climates provide a unique opportunity to reduce the frequency and severity of pedestrian injuries by introducing improvements in vehicle structure and safety devices.

	Methods of literature review

The frequency of pedestrian-vehicle crashes has resulted in many papers providing epidemiological, engineering, and clinical assessments of the problem. Since few of these papers include a multidisciplinary analysis, we have synthesised information from articles that evaluate the severity of the problem, injury aetiology, and biomechanical tolerances in order to provide a scientific basis for current countermeasure designs. In addition to referencing archival and conference publications, we direct readers to a larger general knowledge base of vehicle design principles for safety and corresponding biomechanical studies through website references.

	Anatomy of a pedestrian-vehicle crash

Most pedestrian-vehicle crashes involve frontal impacts, and the vehicle front structures are responsible for most pedestrian injuries (fig 1). In a frontal impact, the chronology of the crash scenario is well documented: the vehicle bumper contacts the lower limbs, the leading edge of the bonnet strikes the proximal lower limb or pelvis, and, finally, the head and upper torso hit the top surface of the bonnet or windscreen (fig 2). In essence, the pedestrian "wraps around" the front of the vehicle until pedestrian and vehicle are travelling at the same speed. During subsequent braking of the vehicle, the pedestrian continues to move forward while the vehicle decelerates. Eventual contact with the ground often produces further pedestrian injuries.⁶

Fig 3.   Distribution of pedestrian injuries in pedestrian-vehicle crashes (left)5 and frequency of injury caused by different vehicle regions (right).9 In adult pedestrians the head and lower limbs are the most commonly injured body parts, with head injury being the main cause of fatality. The windscreen and the bumper are the two main sources of injury in pedestrian crashes.

Despite what seems to be a choreographed sequence of events, the precise trajectory and injuries of a pedestrian depend heavily on the relative sizes of pedestrian and vehicle, the orientation of the pedestrian before impact, and the speed of the vehicle. The relatively low profile of most passenger cars results in an impact below a pedestrian's centre of gravity and produces the pedestrian trajectory shown in fig 2. However, larger or taller vehicles such as multi-purpose vehicles, light trucks, and vans hit a pedestrian at, or above, the centre of gravity.⁷ This impact can result in the pedestrian being projected forwards without contacting the bonnet and then being run over by the decelerating vehicle.

	Injury profiles and countermeasures

Given the preponderance of bumper and bonnet contacts, the head and lower extremities are the most commonly injured body parts.⁸ Since head trauma is responsible for most serious injury and mortality in pedestrians (fig 3), providing a soft impact area for the head is the primary pedestrian safety design. The local stiffness of the vehicle can vary dramatically at different possible impact points for the head. Head impact with the relatively compliant central region of the windscreen generally results in minor to moderate injuries, whereas head contact with the relatively stiff windscreen frame can produce skull and facial fractures, vascular injuries, or diffuse axonal injury even in low speed impacts.¹⁰

Since the bonnet surface is made from sheet metal, it is a relatively compliant structure and does not, by itself, pose a major risk for severe head trauma. However, serious head injury can occur when the head hits a region of the bonnet with stiff underlying structures such as engine components. The solution is to provide sufficient clearance (greater than 10 cm) between the bonnet and underlying structures for controlled deceleration of a pedestrian's head. However, considerations of aerodynamic design and styling can make it extremely difficult to alter a vehicle's front end geometry to provide more under-bonnet space. Active safety systems, such as pyrotechnic devices that rapidly raise the bonnet in a crash (fig 4), can provide the necessary deceleration space during an impact while maintaining traditional bonnet geometry during normal operation of the vehicle.¹¹ As well as providing more deceleration space under the bonnet, the raised rear edge of the bonnet can prevent the head of the pedestrian from hitting the hard area behind the scuttle and the lower windscreen and windscreen frame. More comprehensive coverage can be obtained by using airbags to cover this gap, the lower windscreen, and the stiff structural A pillars on either side of the vehicle (fig 5).

Fig 4.   Pop-up bonnet activated by sensors in the bumper that sense an impact with a pedestrian and rapidly lift the bonnet to provide a softer landing space for the pedestrian's head and upper body. (Reproduced with permission of Autoliv, Stockholm, Sweden)

Fig 5.   Pop-up bonnets are not sufficient to eliminate head contact with the stiff windscreen scuttle and the A pillar, especially in small cars, and windscreen airbags are being developed to cover these stiffer regions. (Reproduced with permission from TRW, Cleveland, OH, USA)

Although head injury is the leading cause of fatalities, lower limb trauma is the most common injury (fig 3).⁸ A direct blow from the bumper usually leads to contact fractures of the tibia or fibula and to damage of the knee ligaments from shearing and bending at the joint. Efforts to reduce the force transmitted to the leg and to increase the area of force application are the primary design considerations. One approach is the application of an extra layer of energy absorbing material over the bumper to add compliance to the relatively stiff conventional structure. However, increasing bumper compliance is limited by the fact that the bumper must also protect the vehicle front from damage in minor vehicle-vehicle collisions.¹²

In experimental tests with cadavers and computer simulations the coupling of moderate alterations in bumper compliance with altered bumper height, geometry, and orientation have shown reduced injury to the lower limbs.¹³ In general, lower bumpers permit the femur and tibia to rotate together as the upper body falls on the car, thus limiting lateral bending of the knee.¹⁴ Bumpers that strike above knee level, common in off road vehicles and multi-purpose vehicles, cause the upper body and the leg to rotate in opposite directions, resulting in severe knee trauma. Deeper bumper profiles and support bars below the bumper can limit the rotation of the leg and provide additional distribution of impact forces. The headlamps and associated housing can also be designed as energy absorbing components to minimise injury.

Fig 6.   "Sub-system impactor" tests developed by the European Enhanced Vehicle-Safety Committee. The tests mimic the effect of a 40 km/h impact with a pedestrian moving laterally across the path of the car.15 The impactors are economical to use and provide a more repeatable test environment than is possible with crash test dummies

	Evaluations of countermeasures

The European Enhanced Vehicle-Safety Committee (EEVC) has developed test specifications and rating systems for assessing the pedestrian injury potential of vehicle front structures. The EEVC recently recommended a series of "sub-system impactor" tests designed to mimic a 40 km/h car-pedestrian impact (fig 6).¹⁵ These tests cover most locations of the vehicle front and involve hitting the bonnet with free flying head forms, the leading edge of the bonnet with a proximal lower limb form, and the bumper with a leg form containing a deformable knee joint. The impactors carry force, displacement, and acceleration sensors whose outputs are interpreted relative to the biomechanical tolerance levels of human head and lower limbs.

A voluntary agreement proposed by European automotive manufacturers stipulates that all new car types introduced after 2010 should comply with EEVC pedestrian safety test requirements. ^{16 17} The New Car Assessment Programs (NCAP) in many parts of the world have already been performing these tests and making the results available to consumers.¹⁸ If vehicles are required to comply with the EEVC recommendations, estimated reductions in pedestrian fatalities should exceed 20%.³ To obtain further casualty reductions, updates of the test procedures will be required. In particular, the windscreen and windscreen frame are not covered by the current EEVC test method but are responsible for more than 15% of all pedestrian injuries⁸ and an even greater share of serious injuries. In addition, improved biofidelity of the impactors and more accurate injury prediction criteria need to be developed through biomechanical studies. ^{19 20} Despite current testing limitations, however, the EEVC methods have increased societal awareness of vehicle design considerations for pedestrian protection and should, when coupled with collision avoidance systems, reduce injuries in what was once believed to be an intractable problem.

	Footnotes

   Competing interests: None declared.

	References

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