Pedestrian and Cyclist Collision Analysis

When a vehicle strikes a pedestrian, the pedestrian is launched approximately at the vehicle's speed (assuming the pedestrian was stationary or moving slowly relative to the vehicle). After launch, the pedestrian follows a projectile trajectory and slides on the road surface before coming to rest. The throw distance from the point of initial contact to the final rest position is measurable from the scene, and it is related to the launch speed through standard projectile and sliding-friction equations.

The Searle equation is the simplest widely used form: V = sqrt(2g × (mu_air × H + mu_slide × d)), where H is the pedestrian's centre-of-mass height, d is the total throw distance including sliding, mu_air is a small aerodynamic drag factor (often set to zero for simplicity), and mu_slide is the friction coefficient for the pedestrian's clothing on the road surface. The Wood and Eubanks variations refine the launch angle and incorporate the pedestrian's body geometry more explicitly. All three methods give a minimum speed at primary contact because they assume the pedestrian was launched at the vehicle's full speed.

The geometry of a pedestrian impact depends heavily on the vehicle's frontal profile. A conventional passenger car with a low hood line produces a characteristic wrap pattern: the leading edge of the bumper strikes the lower limbs (typically around knee height), causing a leg bending injury. The body's centre of mass is above the contact point, so the body rotates forward and upward. The thighs or pelvis then contact the hood, the torso continues rotating, and the head strikes the windshield or A-pillar. The head-impact location on the windshield is called the head wrap distance (HWD) from the bumper, and it is directly related to the vehicle's speed and the pedestrian's height.

SUVs and pickup trucks with high front ends and flat or nearly vertical front faces produce a vault pattern because the primary contact is at the pelvis or torso rather than the lower legs. The body does not rotate over the hood. Instead, it is projected forward as a relatively rigid projectile and lands at a distance that correlates well with vehicle speed. The flat frontal geometry also produces different injury patterns: pelvis and thorax injuries dominate rather than lower limb fractures.

Cyclists present a modification of the pedestrian case. The bicycle geometry raises the rider's centre of mass and changes the initial contact point. The vehicle typically strikes the bicycle first (particularly the front fork or wheel), transmitting force to the rider. The throw trajectory of the cyclist and the deformation pattern of the bicycle frame both carry information about the impact speed and direction, and the same throw-distance equations apply with adjustments for the combined cyclist-bicycle mass.

For a wrap-pattern impact, the location of the head contact mark on the windshield or A-pillar is geometrically linked to the vehicle's speed at impact. At higher speeds, the pedestrian's body acquires more rotational energy and wraps further up and over the hood before the head contacts the vehicle; at lower speeds, the head contacts nearer the base of the windshield or the hood top. The Pedestrian Impact Reconstruction (PIR) method formalises this relationship using the vehicle's known hood length, hood angle, windshield angle, and the pedestrian's measured stature and segment proportions.

In practice, identifying the head contact mark on the vehicle requires careful examination of the windshield and pillar: a star-crack pattern centred on the point of head contact, with possible hair, blood, or tissue deposits. The head wrap distance (the distance along the vehicle's body surface from the bumper to the head mark) is the key measurement. Published PIR data tables and computer simulation tools (such as PC-Crash) then match that measurement to a speed range for the specific vehicle geometry.

A large proportion of serious pedestrian fatalities occur at night. The reconstruction question shifts from pure speed analysis to a paired question: at what distance was the pedestrian first detectable to a driver using the vehicle's actual headlights, and was that distance sufficient for a driver reacting in normal time to stop before impact? Answering the first part requires photometric analysis; answering the second requires combining the detection distance with the time-distance analysis methods covered in the collision reconstruction topic.

Photometric reconstruction involves returning to the scene at night (or simulating the lighting) and measuring the luminance (brightness) of a pedestrian target dressed in the same clothing against the background. The detection threshold is typically taken from published psychophysical research at around 0.5-1.0 foot-lambert contrast or from specific reaction-time data at luminance levels matching the scene. The vehicle's headlight aim and type (halogen, HID, LED) and the presence of glare from oncoming headlights are key variables.

• Dark clothing on a dark background: detection distances can fall below 30 m at low-beam halogen headlights, below some vehicles' stopping distance at moderate speeds.
• Retroreflective clothing: increases detection distance to 80-150 m under the same headlights, typically above stopping distance at urban speeds.
• Glare from oncoming headlights: can temporarily reduce detection distance to near zero during the glare exposure, before vision recovers.
• High-beam headlights: approximately double the detection distance compared to low-beam on a straight road, but high-beam use is precluded by oncoming traffic in many urban situations.

In the United States, pedestrian reconstruction practice is primarily governed by SAE J2980, the AAAM glossary, and the accumulated peer-reviewed literature on throw-distance validation (particularly Backaitis and Woo 1975; Searle 1983; Wood 1991; Eubanks and Hill 1998). Expert testimony is admitted under the Daubert-Kumho standard, meaning the analyst must demonstrate that the specific equations used have been tested and published in peer-reviewed form.

European practice, particularly in Germany and the UK, uses the same underlying kinematic methods but has a stronger tradition of computer simulation using PC-Crash (a commercial multi-body dynamics simulator) to model the full pedestrian trajectory rather than relying solely on simplified throw-distance equations. PC-Crash allows the analyst to vary pedestrian stature, posture, and pedestrian-vehicle friction to produce a sensitivity-tested speed range rather than a single equation-based estimate.

Australian practice under the AS/NZS framework draws heavily on both US and European literature. The Victorian Institute of Forensic Medicine and the police reconstruction units in New South Wales and Queensland routinely use both throw-distance equations and PC-Crash simulation, with experts qualified under the Makita principles (the Australian framework governing admissibility of expert evidence, requiring demonstrated specialised knowledge and an explained intellectual basis, distinct from the US Daubert gatekeeping model).