When a car strikes a pedestrian, the body endures a rapid chain of impacts that can damage nearly every organ system, from shattered leg bones in the first fraction of a second to traumatic brain injury milliseconds later. The severity depends heavily on vehicle speed: a pedestrian’s risk of sustaining a life-threatening injury reaches roughly 50% at around 33 mph and 90% at about 48 mph. But speed is only one variable. The shape of the vehicle, the pedestrian’s age and size, and what happens after the initial strike all determine which parts of the body absorb the worst of it.
The Impact Sequence
A typical collision between a car and an adult pedestrian unfolds in three distinct phases, each lasting only milliseconds but each causing different injuries. In the first phase, the vehicle’s bumper strikes the pedestrian’s legs, usually somewhere between the knee and mid-thigh. Because the center of gravity for a standing adult is near the hips, the lower body is knocked forward while the upper body stays momentarily in place. This causes the torso to rotate toward the hood.
In the second phase, the upper body, now whipping forward, strikes the hood or windshield. The head often hits the windshield or the base of the A-pillar, which is one of the hardest structures on the car’s exterior. Finally, in the third phase, the pedestrian either slides off the vehicle or is thrown clear and hits the ground. At lower speeds, this ground impact can actually be the most dangerous moment for the head, because the body has less forward momentum and may drop almost straight down onto pavement. One study found that in collisions at 20 to 30 km/h, the head injury risk from hitting the ground exceeded the risk from the initial vehicle impact.
How Speed Changes Everything
Speed is the single strongest predictor of how badly a pedestrian is hurt. A large analysis of real-world pedestrian crashes quantified this relationship precisely: the risk of a severe or life-threatening injury (the kind requiring intensive care or surgery) reached 10% at just 17 mph, 25% at about 25 mph, 50% at 33 mph, and 90% at 48 mph. Those numbers underscore why residential speed limits and urban traffic-calming measures focus on keeping vehicle speeds below 25 to 30 mph. The jump from 25 to 40 mph roughly triples a pedestrian’s odds of a devastating injury.
The relationship is not linear. Each additional mile per hour matters more as speed climbs, because kinetic energy scales with the square of velocity. A car going 40 mph carries four times the energy of one going 20 mph. That energy has to go somewhere, and in a pedestrian collision, it goes into the human body.
What Happens to Your Legs
The legs absorb the first blow. The car’s bumper, typically 40 to 50 centimeters off the ground, contacts the tibia and knee region almost simultaneously. The stresses on the knee joint during this impact are extreme. Simulation research shows that the medial collateral ligament, which runs along the inner side of the knee, is usually the first structure to fail, with stresses far exceeding the tissue’s yield point. In real-world terms, that means a torn or ruptured MCL is one of the most common pedestrian injuries, often accompanied by fractures of the tibia or fibula.
Where on the bumper the legs make contact also matters. Research using standardized leg-form testing devices found that impacts near the sides of the bumper, directly in front of the vehicle’s structural side members, caused significantly worse leg injuries than impacts at the center. This is because automakers often place foam or energy-absorbing material behind the center of the bumper to cushion impacts, but the sides, where rigid structural members sit closer to the surface, offer less cushioning. The shape of the vehicle’s front end also influenced ligament injuries specifically. So two vehicles traveling at the same speed can produce very different lower-limb damage depending on their design.
Head and Brain Injuries
Head injury is the leading cause of death in pedestrian collisions, and the mechanics explain why. After the legs are swept and the torso rotates onto the hood, the head typically strikes the upper hood surface, the windshield, or the windshield frame. Biomechanical modeling of this moment shows that when the skull contacts the windshield, it decelerates abruptly while the brain, floating in cerebrospinal fluid, continues moving forward inside the skull. This creates stress concentrations first at the impact site, usually the forehead region, and then on the opposite side of the brain as it rebounds against the rear of the skull. The peak stresses on the skull during windshield impact can exceed the threshold for fracture, and the internal brain tissue experiences pressure waves that can cause contusions, especially in the frontal lobe.
The windshield’s angle plays a role too. Computational studies have found that windshield inclination angles between roughly 32 and 40 degrees tend to produce lower head-injury severity than steeper or shallower angles, during both the initial vehicle impact and the subsequent ground impact. This finding has implications for vehicle design, though the effect is modest compared to the overwhelming influence of speed.
The ground impact deserves its own attention. After separating from the vehicle, a pedestrian’s body rotates in the air before striking the pavement. The angle at which the torso lands largely determines whether the head hits the ground or the shoulder absorbs the blow. Research has shown that the vehicle shape, the pedestrian’s height, and the collision speed all affect whether the pedestrian re-contacts the vehicle before falling, which can actually slow the body’s rotation and reduce ground-impact severity. Children face higher ground-impact head-injury risk partly because their shorter stature changes the rotation dynamics.
Chest, Abdomen, and Spine
Injuries to the torso during a pedestrian collision tend to result from the second phase of impact, when the body strikes the hood, or from being thrown to the ground. The chest can sustain rib fractures, pulmonary contusions (deep bruising of the lung tissue), and in severe cases, traumatic rupture of the aorta. The aorta, the body’s largest artery, is vulnerable to rapid deceleration because it is tethered at certain points inside the chest. When the body decelerates violently, the mobile portions of the aorta continue moving while the anchored portions do not, creating shearing forces that can tear the vessel wall. This injury is frequently fatal.
The abdominal organs are also at risk. The liver and spleen sit in the upper abdomen, partially protected by the lower rib cage, but both are relatively fragile. Experimental measurements on cadaver tissues have shown that the spleen can be injured at accelerations as low as 33 g, and the liver at around 48 g, though the range varies widely depending on age and individual tissue properties. In a high-speed pedestrian collision, the forces transmitted through the torso can easily exceed these thresholds.
Spinal injuries occur in roughly 8% of pedestrians struck by cars, according to a review of over 1,600 cases. Those injuries are spread across the entire spine: about 35% involved the cervical spine (neck), 19% the thoracic spine (mid-back), 37% the lumbar spine (lower back), and 27% the sacrum (base of the spine). Almost all patients with spinal injuries also had injuries elsewhere. The pattern suggests a “double impact” mechanism, where the initial vehicle strike and the subsequent ground impact each damage different spinal regions.
Your Body’s Immediate Emergency Response
Within seconds of a major impact, the body mounts a massive physiological response. If there is significant bleeding, whether visible from open wounds or hidden inside the abdomen or chest, the drop in circulating blood volume triggers traumatic shock. Blood pressure falls, the heart races to compensate, and the body begins shunting blood away from the skin and extremities toward the brain and vital organs. This is why trauma patients often appear pale, clammy, and confused even before anyone has assessed the full extent of their injuries.
In the worst cases, severely injured patients can spiral into a dangerous feedback loop that trauma surgeons call the “lethal triad”: hypothermia, acidosis, and coagulopathy. The body’s temperature drops because of blood loss and exposure. Reduced blood flow causes tissues to switch to anaerobic metabolism, producing lactic acid and making the blood increasingly acidic. And both the cold and the acidity impair the blood’s ability to clot, which worsens bleeding, which worsens the other two problems. This combination is present in roughly 10 to 34% of severely injured patients depending on how badly they are hurt, and it carries a grim prognosis. Recognizing and interrupting this cycle is a central goal of trauma surgery, which is why emergency teams prioritize warming the patient, stopping hemorrhage, and replacing blood products rather than attempting definitive repairs of every injury at once.
Why Vehicle Shape Matters
Not all vehicles pose the same threat to pedestrians traveling at the same speed. The height and shape of the front end change which parts of the body are struck first and how the body moves after impact. In a collision with a traditional sedan, the low bumper strikes below the knee, the pedestrian’s legs buckle, and the upper body rotates onto a relatively low, sloping hood. With an SUV or pickup truck, the higher bumper and blunter front end strike the pedestrian closer to the pelvis or abdomen, which tends to push the body forward rather than rotating it upward.
Simulation research using detailed human body models has shown that pickups produce the most severe chest injuries, with rib fracture risk reaching 100% at moderate speeds, driven by direct contact between the hood’s leading edge and the chest. SUVs generated the highest pelvic forces, causing pubic bone and sacroiliac joint injuries, while sedans produced lower severity in both areas. Separately, studies of head injury found that SUVs produced higher head-injury scores than sedans at equivalent speeds, but for a counterintuitive reason: the head-to-hood contact force was actually slightly lower in SUV impacts, yet high neck tension from the body’s different rotation pattern increased overall head-injury severity.
These findings are driving changes in vehicle design. Automakers have increasingly redesigned hood structures with deformable reinforcements and greater clearance between the hood surface and the rigid engine components underneath, aiming to create a cushioning zone that absorbs energy during a pedestrian’s head impact. Some vehicles now include active hood systems that pop the hood upward in the instant before a pedestrian collision, increasing that clearance further.
When the Pedestrian Is a Child
Children experience different injury patterns than adults, partly because of their smaller stature. A vehicle bumper that strikes an adult’s knee will hit a young child at hip or abdominal level, directing more force into the trunk. A classic teaching in trauma medicine, known as the Waddell triad, describes the expected combination of injuries in a child struck by a car: a femur fracture on the side of impact, an internal chest or abdominal injury on the same side, and a head injury on the opposite side (from the secondary ground impact). In theory, the child’s leg is broken by the bumper, the torso is injured by the hood, and the head is hurt when the child falls and the head whips in the opposite direction.
In practice, though, the full triad is quite rare. A study of 465 pediatric pedestrian-versus-vehicle cases found that only 2.4% had all three components, and just one patient out of the entire cohort had the exact injury pattern Waddell originally described. Head injuries were common, occurring in about half the children, and the combination of head and leg injury was seen in roughly 17%, but the classic three-part pattern was the exception. The lesson is not that children are safer than feared, but that injury patterns are more variable than the textbook suggests, and clinicians need to assess every body region regardless of what the expected pattern predicts.
Older Adults Face Worse Outcomes
At the other end of the age spectrum, elderly pedestrians are especially vulnerable. Comparative studies have shown that older patients sustain higher rates of fractures and intracranial injuries than younger adults struck at similar speeds, and their mortality is significantly worse. Several factors converge: aging bones are more brittle and fracture at lower forces, the brain atrophies slightly with age leaving more room for it to move inside the skull during impact, blood vessels become more fragile, and the body’s ability to mount a physiological response to shock declines. Older adults are also more likely to be on blood-thinning medications, which magnifies the danger of any internal bleeding.
Run-Over and Crush Injuries
Not every pedestrian collision follows the bump-rotate-fall sequence. In some cases, the pedestrian is knocked down and then run over by the vehicle’s wheels. This produces a different and particularly severe injury pattern. A review of crush injuries at a major trauma center found that being run over by a vehicle was the most common mechanism, accounting for more cases than all other crush scenarios combined. Fractures were the most frequent acute injury, but the real danger lies in complications that develop over the following hours. Rhabdomyolysis, in which crushed muscle tissue releases proteins that overwhelm the kidneys, occurred in a substantial proportion of cases, and compartment syndrome, where swelling within a limb’s tissue compartments cuts off blood flow, required emergency surgery.
Compartment syndrome can develop with a dangerous delay. In one documented case, a patient with a crush injury from a car wheel appeared stable initially, with intact sensation and blood flow to the foot. Five hours later, blood flow had been cut off by progressive swelling, and emergency surgical release of the tissue compartments was needed to save the limb. This delayed onset is part of what makes crush injuries treacherous: a limb that looks salvageable in the emergency room can deteriorate rapidly if the warning signs are missed.
Physical Recovery Can Take Years
Surviving a pedestrian collision is only the beginning. Research tracking road-crash patients with orthopedic injuries found that problems with daily activities were most common at six months after injury, and pain was the dominant complaint at twelve months. These timelines surprise many patients, who expect a more linear recovery. The persistence of pain and functional limitation well past the point when bones have healed and surgical wounds have closed is one of the most frustrating aspects of recovery.
Qualitative research with major trauma survivors three to five years after their injuries found that many still experienced significant restrictions on physical activity. Fear of re-injury, ongoing pain, and fatigue were the primary barriers. Participants described frustration and distress over the impact on their social lives, family roles, and overall enjoyment of life. Weight gain and declining fitness from enforced inactivity created secondary health concerns that compounded the original injury’s effects.
The Psychological Aftermath
Post-traumatic stress disorder is common after serious vehicle collisions. A prospective study of crash survivors found that PTSD developed in about 28% of patients by six months and persisted in roughly 24% at one year. Women, people with a prior history of depression, and those whose crash involved a fatality were at higher risk. Interestingly, people who sustained a traumatic brain injury in the crash were not more likely to develop PTSD overall compared to those without brain injury, though their symptom profile differed in the early weeks.
Specifically, brain-injured patients reported fewer intrusive memories and less intense feelings of fear and helplessness in the first month after the crash, likely because impaired consciousness during the trauma reduced the formation of vivid traumatic memories. By three months, though, the PTSD symptom profiles of brain-injured and non-brain-injured patients converged. The practical implication is that clinicians should not assume a head-injured patient is psychologically protected just because they do not recall the event clearly. Screening for PTSD matters regardless of whether the patient remembers the impact.
The combination of chronic pain, physical limitation, and psychological trauma creates a recovery landscape that is far more complex than the initial surgical treatment might suggest. Many survivors describe the first year after their injury as a period of grieving for their former physical capabilities, even as their medical team considers the acute treatment a success.