Introduction

Death by motor vehicle crash/collision (MVC) is currently the fifth leading cause of death for all Americans [1]. The American Automobile Association reported that the economic costs associated with traffic crashes were $299.5 billion, based on 2009 data [2]. Over the past decade, vehicle-related death and injury rates have steadily decreased. In 2002, the occupant fatality rate per 100 million vehicle miles of travel (VMT) was 1.51; from 2010 to 2011 it was 0.98/VMT – a historic low. Despite this improvement in fatality rates, there were still over 5 million MVCs in the US during 2011. These crashes resulted in about 2.2 million injuries and just over 32,000 deaths [3].

Effect on EMS

In the US, someone dies in an MVC every 16 minutes [3]. Moreover, fatal crashes are far outnumbered by crashes with non-fatal but serious injuries requiring urgent medical attention. Therefore, a significant percentage of EMS calls relates to MVCs. While many crashes involve only single vehicles, there may be multiple occupants in that vehicle. In a crash involving more than two vehicles, many more EMS resources may be required. Likewise, if public transportation, such as trains or buses, is involved, the need for those resources is greatly expanded.

The hazards present at the scene of the MVC are many and varied. Continued high-speed traffic flow on freeways represents a significant threat to the safety of the rescue crews. Spilled fuel can result in a fire risk and broken glass and sharp metal edges can also prove hazardous to the unprotected EMS provider. Grounded power lines and hazardous materials being transported on the roadways present additional risks.

Emergency medical services physicians and providers may not have the personal protective gear to operate in such environments. Fire department response is often required to address hazardous issues. Rural or smaller agencies may not have the heavy extrication equipment necessary to access trapped patients. Regardless of whether the EMS system is fire department based, additional resources (e.g. rescue, law enforcement) are often used at the scene of an MVC.

Motor vehicle crash injury biomechanics

During a crash, different parts of an occupant’s body are subjected to sudden acceleration or deceleration. Injury results when tissues are disrupted by local concentrations of physical force generated by a crash event. Morbidity and mortality occur when vital organs absorb energy beyond their tolerance. The ability to tolerate physical forces varies considerably from individual to individual. Clinically, it is important to remember that those at either end of the age spectrum (children and the elderly) are particularly sensitive to serious injury due to the reduced ability of their tissues to absorb energy.

Acceleration and deceleration are defined as changes in velocity over time, measured in G forces (the weight of objects in earth’s gravitational field). In healthy, non-elderly, adult individuals, the upper end of transient G force that can be tolerated is about 30 G [4]. Both speed and stopping distance contribute to the overall G force experienced during an MVC. Gs increase by the speed squared, while doubling the stopping distance cuts the Gs by half. Automotive safety equipment such as seat belts, airbags, and vehicle deformation (crumple) zones effectively increase the stopping distance for the vehicle occupant during a crash.

Four possible collisions occur during a MVC.

1. The vehicle strikes another object and the crushing of the vehicle’s structure absorbs energy. Vehicles with a greater capacity to deform absorb more energy and effectively increase the stopping distance. During this phase occupants continue in motion.
   
2. A second collision occurs as a passenger strikes the vehicle’s interior or restraint system such as seat belt and airbags.
   
3. Internal organs continue in motion until they strike surfaces such as the skull or chest wall.
   
4. Loose objects and unrestrained occupants can strike the person, resulting in further injury [4].

Poor automotive design, absence of airbags, and lack of seat belt use decrease the stopping distance and thereby increase the G force experienced by a vehicle occupant.

Safety restraints

Inertia causes occupants of a car to be moving at the same speed as the car. In a crash, the car comes to an abrupt halt and the occupants inside continue at the speed the car was traveling until their bodies are stopped by objects such as the windshield, instrument panel, or steering wheel, if they are not wearing seat belts. Stopping an object’s momentum requires force acting over a period of time.

A seat belt, anchored to the vehicle, is designed to apply the stopping force to more durable parts of the body over a longer period, allowing the occupant to “ride down” the crash as the vehicle crumple zones crush and absorb the energy, and helps protect the body from serious injuries. In contrast, unrestrained occupants do not get the additional “ride down” time and, at higher speed, hit the vehicle interior with weaker parts of the body, e.g. upper abdomen to steering wheel. The use of three-point seat belts reduces the risk of fatal injury in passenger car occupants by 45% and the risk of moderate-to-critical injury by 50%. The risks are reduced by 60% and 65% respectively for light truck occupants [5].

Airbags spread the force required to stop the occupants over a large part of their bodies, minimizing local concentrations of force that can disrupt tissues and cause injury. The airbag has the space between the passenger and the frontal or side components of a vehicle and a fraction of a second in which to work. Even that tiny amount of space and time is valuable, however, if the system can slow the passenger evenly rather than forcing an abrupt halt to the occupant’s motion. The National Highway Traffic Safety Administration estimates that belts alone reduce fatalities by 45%, and when combined with airbags, there is a 50% overall reduction in mortality [5].