20-12-2012, 04:46 PM
Lives Saved Calculations for Seat Belts and Frontal Air Bags
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Introduction
Seat belts and frontal air bags are among the most important safety devices in society today, together saving thousands of lives each year. The National Highway Traffic Safety Administration quantifies the benefits of these devices by estimating the numbers of people saved by each device, the number who would have lived if more occupants had buckled up, and the corresponding expressions of the savings and loss in financial terms. This information is then used in NHTSA literature and speeches to promote belt use, and is used to perform cost-benefit analyses of proposed regulations concerning belts and bags.
In 2007, an estimated 15,147 lives were saved by seat belts, and 2,788 lives were saved by air bags. If seat belt use increased to 100 percent, then an additional 5,024 lives would have been saved (Starnes, 2008). When these seat belt numbers are added together to equal 20,171 (= 15,147 + 5,024), this estimate is often referred to as “potential lives saved” or “lives savable.”
This report concerns the calculation of these estimates of lives saved by belts and air bags, and the lives savable if more occupants had used their belts. For information on the calculations of cost savings see Blincoe et al., 2002.
Terminology
In the remainder of this report, the term air bag will always mean “frontal air bag,” although we occasionally write frontal air bag for emphasis. The term vehicle will refer to passenger vehicles.
Lives saved by belts will mean “passenger vehicle occupants of age 5 and older saved by belts,” and lives saved by bags will mean “passenger vehicle occupants 13 and older saved by frontal air bags.”
In the context of potential lives saved, belt use will mean “front outboard belt use during daytime.” Lives savable if belt use had been x percent will mean “passenger vehicle occupants over 4 years old who would have been saved if (daytime front outboard) belt use had been x percent.” We shall use the terms “potential lives saved,” “lives savable,” and “lives potentially saved” interchangeably to refer to this quantity.
Fatalities Used in the Calculations
Lives saved and potential lives saved are calculated from a list of the numbers of fatalities in a given year, broken out by the various restraint systems in the vehicle and that were used. This section describes how this list, which we call the fatality counts, is produced. The counts will comprise any person 5 and older who died within 30 days of being in a crash in a specified year, who was in a passenger vehicle in a location where there was a belt (e.g., not in the bed of a pickup).
Data Source
NHTSA compiles a census of all motor vehicle fatalities in the United States from police reports called the Fatality Analysis Reporting System (FARS). A fatal crash is defined as a police-reported crash involving a motor vehicle in transport on a public road, street, or highway in which at least one person, called a fatality, died within 30 days of the crash. This section describes the information we extract from FARS for the fatality counts. It also describes the treatment of unknowns (e.g., when we do not know the age of an occupant who died).
The fatality count is produced using the following FARS variables: body type (BODY_TYP), make and model (MAKE_MOD), VIN-derived model (VINA_MOD), vehicle identification number (VIN), towed trailing unit (TOW_VEH), VIN-derived truck series (SER_TR), model year (MOD_YEAR), person type (PER_TYP), age (AGE), seating position (SEAT_POS), restraint use (REST_USE), and injury severity (INJ_SEV).
Note that multiple model information (MAKE_MOD, VIN, VIN model, VIN truck series) is used. These are used by Kahane (2000) to determine the type of seat belts in a vehicle and whether the vehicle has air bags.
Inclusion Criteria
The fatality counts reflect the people in FARS in a given data year who meet all of the following criteria. These criteria identify passenger vehicle occupants 5 and older who died from motor vehicle crashes and had access to belts where they were seated, regardless of whether the belts were used.
Inclusion Criteria
1.The person was an occupant.
2.The person died within 30 days of the crash.
3.Either (a) the person was over age 4 at the time of the crash, or (b) age was unknown and the child was not in a child safety seat.
For a Sole Safety Device
We first explain the relatively simple case of the effectiveness of a solitary safety device. Section 4.2 presents the more complex scenario of multiple devices designed to protect occupants in a common setting, such as belts and bags protecting people in crashes.
Devices and Settings
In general, to estimate lives saved, one must specify a potentially life-threatening situation, called the setting, and one or more factors, called devices, that affect survival. In this report, the setting is the crash of a passenger vehicle and the devices are seat belts and air bags.
In the “single-device scenario” (or “sole-device scenario”) there is only one device. We use this simplified situation to introduce the basic concepts of benefits calculations, including effectiveness ratings, potential fatalities, lives saved, and potential lives saved (or savable lives). We then expand these concepts to the more complex two-device scenario used in our calculation for seat belts and air bags.