In pedestrian accidents, the many differences that occur in the impact location and size of pedestrians and design of vehicles produce wide variations in the severity and location of the resulting injuries.

To obtain a better understanding of the mechanisms influencing the variability of these injuries, B. L. Cars PLC and the Transport and Road Research Laboratory have conducted a collaborative project using highly developed computer simulation techniques, matched to experimental test results, to study the relationship of vehicle shape and stiffness with the resulting forces and accelerations acting on pedestrian dummies.

Changes to the spoiler, bumper, and bonnet leading edge for simulated impacts to adult pedestrians at a speed of 40km/h show the following results:​

1. A low front profile concentrates most of its input energy well below the centre of gravity of an adult, inducing high angular rotation onto the bonnet resulting in high head impact velocities.
2. Increasing the height of the bonnet leading edge increases the energy it absorbs resulting in higher accelerations to the regions of the adult and child that it contacts, but this is combined with reduced head imnact velocities.

3. In a 40km/h impact, the energy absorbed by the bumper or bumper plus spoiler is generally between 1,000 and 1,200J.

   A bumper mounted at 450mm above the causess high bending moments and shear forces adult knees. f the bumper stiffness is above3 kN, child pelvic accelerations may also be high.

   Lowering the bumper reduces the risk to the adult knee and child pelvis. Introducing a suitable spoiler (air dam) reduces the risk of adult knee and lower leg injury and gives the most satisfactory conditions, providing the bumper component of the bumper/spoiler stiffness is approximately 3 kN.

4. The input energy of the bonnet edge and grille varies widely with respect to its location, increasing as the bumper lead decreases or as the bonnet leading edge height increases.

   For adult impact at 40km/h, energy input values for the bonnet front ranged from 150J for a low bonnet with a bumper lead of 150mm, increasing to 1,700J for an 850mm-high vertical front profile and reducing to 600J for a high profile and 350mm lead. For some car proliles, a stiffness of 7kN, typical of some current small cars, produces high child pelvic accelerations and high adult femur bending moments. Reducing the stiffness to 5kN gives acceptable child pelvic accelerations and reduces the magnitude of leg bending moment by about 20 percent.

5. Within thw range of shapes studied, the adult head impact velocity may be reduced by up to 40 percent of the worst case by increasing the bonnet height from 600 to 850mm. It may also be reduced by 30 percent by reducing the bumper lead from 150mm to zero, but only for a medium-height bonnet. Extreme values of head impact velocity range from 40 percent above car impact velocity to 25 percent below.

   For the child, head impact velocities are more consistently just below car impact speed, reducing by 60 percent for a high vertical front profile.

6. 6. The most significant changes noted in the simulations of different dummy sizes are that head impact velocity increases in magnitude and the location of head impact point moves further rearward with increasing dummy height.

   The severity of pelvic acceleration is increased as the pedestrian's height is reduced because the bonnet leading edge strikes closer to the pelvis.

7. The relationships determined in these studies have been utilised to give guidelines for modifying a small saloon car to give improved pedcstrian protection(2).