A large body of evidence shows that SUVs represent a significantly greater hazard to pedestrians than do ordinary cars. Analysis of data from the US fatal accident reporting system database (FARS) shows that, for the same collision speed, the likelihood of a pedestrian fatality is nearly doubled in the event of a collision with a large SUV compared with a passenger car.

Other studies have uncovered rates of severe injury and death of up to four times for pedestrians in collisions with SUVs.

The danger is not the weight of the car, but the design and height.

A common misconception is that the increased vehicle mass of SUVs is responsible for the increased hazard to pedestrians. In fact, although vehicle mass is important in car to car collisions, it is a minor factor in vehicle-pedestrian collisions given the disparity between the weights of the pedestrian and of the vehicle.

The increased mortality and morbidity from SUVs arises primarily from the geometry of the front end structure. In a typical collision between a car and an adult, the bumper strikes the lower legs and the leading edge of the bonnet strikes the femur/pelvis, causing the pedestrian to rotate towards the bonnet. This results in the bonnet or windscreen hitting the shoulders or head. After this further injuries often occur through impact with the ground. A key mitigating factor in injury severity is the relatively peripheral nature of the primary impact of the bumper to the lower legs. This affords some protection to the critical upper body regions in the primary impact, and the resulting body rotation on to the bonnet tends to further diminish the impact—a characteristic often called “wrap and carry.” The principal pedestrian injuries from cars are predominantly fractures of the tibia and fibula and knee injuries from the primary impact with the bumper and head injuries from the secondary impact with the bonnet or windscreen.

SUV bonnets are higher than those of cars and this results in a more severe primary impact on the critical central body regions of the upper leg and pelvis. Also, there is less rotation as the impact is closer to the body centre of mass, resulting in a more efficient transfer of energy. For example, raising the bonnet leading edge height from 600 mm to 850 mm increases the impulse by a factor of about two. This results in a doubling of injuries to vulnerable regions such as the head, thorax, and abdomen.

Elderly pedestrians are more at risk because they are weaker, less agile and may have poorer reactions which may make them less likely to avoid being struck and more at risk of suffering serious injuries and dying.

Young children are endangered as well, due to the difficulty of drivers seeing them in front of or around the vehicle.

Factoring all this in, the Irish Medical Association recently called on motor manufacturers and distributors to display warning notices on SUVs that advise potential purchasers of the increased risk of severe injury and death to pedestrians associated with these vehicles.

Resistance from the industry to such initiatives is likely to be strong, just as it has been from the tobacco industry for warnings on cigarettes. Nevertheless, healthcare advocates should take heart from previous successful traffic safety initiatives. Addressing the hazards posed by SUVs to pedestrians is an emerging and real traffic safety challenge in the developed world.

It is a challenge with which the US has already been confronted, and around which there has been a large amount of research for decades, and not just into pedestrian fatality rates. As Keith Bradsher chronicles in great detail in his 2002 book, High and Mighty, SUVs inflict catastrophic damage on cars they hit (essentially transferring the injury and fatality risk onto the other driver) as well as posing a lethal threat to pedestrians. They also roll over too easily, killing and injuring occupants at an alarming rate.

(Bradsher was the Detroit bureau chief of the New York Times from 1996 to 2001, during which time he won the George Polk Award and was a finalist for the Pulitzer prize.)

Resources:

• “Sport Utility Vehicles and Older Pedestrians”, BMJ 2005;331:787-788 (8 October), doi:10.1136/bmj.331.7520.787

• High and Mighty. SUVs: The World’s Most Dangerous Vehicles and How They Got That Way, Keith Bradsher, 2002