Effectiveness Of Seat Belts Research Articles

Effect of Seatbelt and Airbag on High-Energy Traffic Accident

Effect of Seatbelt and Airbag on High-Energy Traffic Accident

The fatal injuries of car drivers

We often refer to road fatalities without knowing exactly what injuries are responsible for them. Based on the Rhône Road Trauma Registry this paper sets out to describe the topography, nature and frequency of the fatal injuries sustained by car drivers. Mean annual mortality at the wheel of a car, computed by dividing the total number of drivers killed ( n = 383) by the population of the Rhône Département (1.6 million) during the period 1996–2004 was 5.41 males per 100,000 and 1.41 females per 100,000, with 78% of the casualties residing in the Département. A reduction has been observed since 2003. Three-quarters of the casualties died at the scene of the crash. The results confirm the effectiveness of seat belts. The observed lethality was 0.43% for unbelted drivers and 2.7% for belted drivers (RR = 0.16 [0.12; 0.21]). The injuries were analyzed for the 287 killed drivers whose deaths could be explained by the described injuries (at least one AIS 4+ injury). Of these, 41% had an ISS of 75 (at least one AIS 6 injury), 21% had an ISS of between 40 and 74, 33% an ISS of between 25 and 40, and 6% an ISS of between 16 and 24. In the case of all the AIS 4+ injuries, the three most frequent locations for injuries were the thorax only (30% of casualties), the head only (23%) and a combination of the two (18%). Abdominal injuries occurred in only 10% of casualties and spinal injuries in 9% of casualties. In the thorax, the most common injury was flail chest with haemothorax or pneumothorax. In the case of the head, the most frequent injuries were to the brain (haemorrhage, haematoma and axonal injuries). Complex fractures of the base of the skull were the second most common craniocephalic injuries. In spite of the use of restraint devices, the thorax and head are still the priority vital areas for protection in the case of car drivers. For one in four of the fatalities, death cannot be explained by any of the injuries we know about. As road traffic accidents are considered to be a cause of death in their own right, autopsies are rarely performed in France on such fatalities. This means we do not know about any injuries that cannot be detected by an external examination of the cadaver.

810 妊婦自動車運転手に対するシートベルトの有効性について(S02-3 インパクトバイオメカニクス(3) 子供と妊婦の傷害,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

To examine the effect of seatbelt using for pregnant women drivers, sled tests were performed. Changes of intra-uterine pressures in anthropometric dummies representing pregnant women were examined. At the low speed of frontal collision, seatbelt reduced the forward movement of the dummy and subsequently decreased peak intra-uterine pressure by 55% to 65%. At rear-end collision with 6.5 G of acceleration, seatbelt also reduced the forward movement of the dummy due to rebound and prevented the second impact between steering-wheel and the abdomen. This traumatomechanical study using pregnant dummy with varying impact dynamics is useful for improving the seatbelt use in pregnant women drivers.

Rollover Injury: Effects of Near- and Far-Seating Position, Belt Use, and Number of Quarter Rolls

Purpose. Vehicle and occupant responses in rollovers are complex since many factors influence both. This study analyzes the following factors: 1) belt use, 2) seated position with respect to the lead side in the rollover, 3) another front occupant in the crash, and 4) number of quarter rolls. The aim was to improve our understanding of rollover injury mechanisms. Method. Rollover accidents were analyzed using 1992–2004 NASS-CDS data. The sample included adult drivers and right-front passengers. All occupants were evaluated and then a subset of non-ejected occupants was analyzed. Using roll direction and seating position, the sample was divided into near- and far-seated occupants. Injury and fatality risks were determined by seatbelt use, occupancy, rollover direction, and number of quarter rolls. Risk was defined as the number of injured (e.g., MAIS 3+) divided by the number of exposed occupants (MAIS 0-6). Significance in differences was determined. A matched-pair analysis was used to determine the risk of serious injury for near- and far-seated occupants who were either belted or unbelted in the same crash. Results. For all occupants, serious injury risks were highest for far-seated, unbelted occupants at 18.1% ± 4.8%, followed by near-seated unbelted occupants at 12.0% ± 3.5%. However, the difference was not statistically significant. Belted near- and far-seated occupants had a similar injury risk of 4.3% ± 1.2% and 4.0% ± 1.2%, respectively. For non-ejected occupants, serious injury risk was 9.5% ± 3.2% for far-seated unbelted occupants and 4.9% ± 2.1% for near-seated unbelted occupants, not a statistically significant difference. Serious injury risk was similar for belted near- and far-seated non-ejected occupants, at 3.6% ± 1.1%. Seatbelts were 64.2%–77.9% effective in preventing serious injury for all occupants and 62.1%–26.5% for far- and near-seated, non-ejected occupants, respectively. Based on the matched pairs, seatbelts were less effective for near-seated (5.0%) compared to far-seated (2.8%) occupant MAIS 3+F risks. This was similar for non-ejected occupants. An unbelted near-seated occupant increased the risk for a belted far-seated occupant by 2.2 times, whereas an unbelted far-seated occupant increased the risk for a belted near-seated occupant by 10.2 times. For all occupants, the risk of serious injury increased with the number of quarter rolls, irrespective of seated position. For near-seated occupants, seatbelt effectiveness was higher in ≤1 roll than 1+ roll, at 72.3% compared to 28.3%. For far-seated occupants, seatbelt effectiveness was similar in ≤1 and 1+ roll samples at 78.3% and 76.8%, respectively. Near-seated occupants had the lowest serious injury risk when they were the sole occupant in the vehicle. This was also true for non-ejected occupants. However, far-seated occupants had a lower injury risk when another occupant was involved in the crash. Conclusions. The effect of carrying another occupant appears to reduce the risk of serious injury to far-seated occupants. However, near-seated occupants are better off being the sole occupant in the vehicle. Seatbelt effectiveness was lowest at 28.3% for non-ejected, near-seated occupants in 1+ rolls. This finding deserves further evaluation in an effort to improve seatbelt effectiveness in rollovers. For belted drivers alone in a rollover, fatality risks are 2.24 times higher for the far- versus near-seated position. Analysis of rollovers by quarter turns indicates that occupants are both far-side and near-side in rollovers. The extent to which this confounds the relationship between roll direction, seating position, and injury risk is unknown.

SH14 COLONEL (DR.) JOHN PAUL STAPP – CRASH TEST DUMMY, TRAUMA PREVENTION PIONEER & MURPHY’S LAW

Dr John Paul Stapp was not only the “fastest human on earth;” he was also the quickest to stop. In 1954, Dr Stapp attained a then world record land speed of 632 mph (1020 km/h), going from a stand still to a speed faster than a .45 bullet in 5 s on a rocket sled, and then to a dead stop in 1.4 s, all in the name of safety.Stapp pioneered research on the effects of mechanical forces on living tissues during aircraft crashes. Stapp planned a series of tests on humans and set out to develop a harness to hold them. First, he used a dummy to perfect the harness and after 32 sled runs, he was ready to try it out on a human crash test dummy, himself. By 1954, Stapp had volunteered for 29 deceleration and windblast experiments. On his last run he sustained more than 40 g’s. He suffered fractured ribs and wrist, and retinal haemorrhages which left him temporarily blind.Whilst at the Air Force Stapp also began a car‐crash study program to investigate fatalities in auto accidents. From this program, safety recommendations for the car industry were established including fastening seats more securely to the car body, improving bumper design and proving the effectiveness of seat belts. Stapp’s work resulted in the 3‐point restraint system of today’s belts which has saved millions of lives.Stapp is also credited with coining one of the most famous phrases in history. One of his assistants, Capt E. Murphy, rigged a harness incorrectly yielding no data on the forces Stapp was subjected to. After discovering what happened, Stapp observed, “If there are two or more ways to do something and one of those results in a catastrophe, then someone will do it that way.” It has been called “Murphy’s Law” ever since.

Incorrect Terminology in: Effectiveness of Child Safety Seats vs Seat Belts in Reducing Risk for Death in Children in Passenger Vehicle Crashes

Incorrect Terminology in: Effectiveness of Child Safety Seats vs Seat Belts in Reducing Risk for Death in Children in Passenger Vehicle Crashes

Statistical adjustment for misclassification of seat belt and alcohol use in the analysis of motor vehicle accident data

The use of seat belts and alcohol is often misreported in police motor vehicle accident reports for a number of reasons. To avoid penalties, occupants often over report seat belt use and under report alcohol use. Police officers sometimes fail to account for evidence such as presence of belt burn, condition of belts, odor of alcohol, crash patterns, etc. Biased conclusions result when using misclassified accident data to estimate the effectiveness of seat belts in preventing injuries and reducing medical costs. We investigated the effects of misclassification of seat belt and alcohol use on the odds ratio of injury as well as medical costs. A statistical method and a SAS program were developed to adjust odds ratios of injury and medical cost estimates to account for misclassification of seat belts and alcohol use. The method allowed for incorporation of variables that could affect misclassification of seat belt and alcohol use. We conducted a Monte Carlo simulation and found that when there were large differences between the misclassification rates for major and minor injury, the unadjusted odds ratio could have up to a 90% bias while our adjusted odds ratio was effectively unbiased. To illustrate the method, we estimated the misclassification rates of seat belt and alcohol use by comparing merged police and hospital reports from Nebraska motor vehicle accident data sets (1996–1997) and then evaluated the bias of the odds ratio of injury and medical costs estimates due to misclassification. Our results showed that the bias of the odds ratio of injury and medical costs due to misclassification of seat belts and alcohol use depended both on the amount of misclassification and the reported frequencies. Misclassification about seat belt and alcohol use only slightly biased the unadjusted odds ratio estimates and mean hospital charge, while misclassification resulted in approximately a 69% underestimate of the total medical costs savings due to seatbelts. However, due to the small size of the merged Nebraska police and hospital data set used to estimate misclassification rates, these results are likely somewhat imprecise.

Effectiveness of Child Safety Seats vs Seat Belts in Reducing Risk for Death in Children in Passenger Vehicle Crashes

To provide an estimate of benefit, if any, of child restraint systems over seat belts alone for children aged from 2 through 6 years. Cohort study. A sample of children in US passenger vehicle crashes was obtained from the National Highway Transportation Safety Administration by combining cases involving a fatality from the US Department of Transportation Fatality Analysis Reporting System with a probability sample of cases without a fatality from the National Automotive Sampling System. Children in tow-away [corrected] crashes occurring between 1998 and 2003. Use of child restraint systems (rear-facing and forward-facing car seats, and shield and belt-positioning booster seats) vs seat belts. Potentially confounding variables included seating position, vehicle type, model year, driver and passenger ages, and driver survival status. Death of child passengers from injuries incurred during the crash. Compared with seat belts, child restraints, when not seriously misused (eg, unattached restraint, child restraint system harness not used, 2 children restrained with 1 seat belt) were associated with a 28% reduction in risk for death (relative risk, 0.72; 95% confidence interval, 0.54-0.97) in children aged 2 through 6 years after adjusting for seating position, vehicle type, model year, driver and passenger ages, and driver survival status. When including cases of serious misuse, the effectiveness estimate was slightly lower (21%) (relative risk, 0.79; 95% confidence interval, 0.59-1.05). Based on these findings as well as previous epidemiological and biomechanical evidence for child restraint system effectiveness in reducing nonfatal injury risk, efforts should continue to promote use of child restraint systems through improved laws and with education and disbursement programs.

Case study: 128 injured in rollover coach crashes in Sweden—Injury outcome, mechanisms and possible effects of seat belts

The risk for injuries in rollover coach crashes are dependent on whether the occupants are belted or not. However, the influence of the different belt systems for reducing injuries has remained unclear. Since many injuries sustained are caused by impacts with the interior, passenger interactions or ejection through a window, the advantages by proper seat belt systems are evident. In this study, representing the most common serious crash scenario for serious injury, 128 injured in rollover cases were analysed with regard to the injury outcome, mechanisms and the possible injury reduction for occupants when using a safety belt. Furthermore, the different belt systems were compared to explain their contribution to increased safety. Based on medical reports and questioning of the passengers, the injuries sustained are recorded according to the AIS classification. The next step was the identification of the injury mechanisms, using the passenger statements as well as results from numerical occupant simulations. It is important to mention that this study was purely focused on detection of the injury mechanism to avoid the reported injuries. The possibility of additional injuries due to the wearing of a belt were not taken into account. However, the analysis of the 128 injured showed a considerable increase in safety for belted occupants through limiting interior contacts, minimising passenger interaction and reducing the possibility of ejection.

Effectiveness of safety belts and Hierarchical Bayesian analysis of their relative use

Seat belts are effective devices for both preventing serious injury and reducing the likelihood of a fatality. Police crash data made available from the Emirate of Dubai in the United Arab Emirates are utilized to evaluate the effectiveness of safety seat belts and to propose a methodology to assess levels of the device use based on demographic factors. Analysis is based on rates and standardized rates. However, rates based on data disaggregated to small levels tend to be unreliable due extreme values resulting from small counts. A Hierarchical Bayesian approach is employed to smooth the standardized safety belt use rates (SBR). Extreme SBRs are shrunken towards an overall mean. Results of the analysis indicate that restrained occupants are less likely to incur fatal or serious injuries compared to the unrestrained. There is no significant difference between ethnic groups (local/expatriate) living in Dubai in terms of level of compliance with belt use.

Method to evaluate the effect of safety belt use by rear seat passengers on the injury severity of front seat occupants

Although the effectiveness of seat belts for reducing injury to rear seat passengers in traffic accidents has been well documented, the ratio of rear-seat passengers restrained by seatbelts remains lower than that of drivers or passengers in front seats. If passengers in rear seats do not wear seat belts, they may sustain unexpected injury to themselves when involved in accidents, and also endanger front occupants (drivers or front seat passengers). This paper focuses on the tendency of front seat occupants to sustain severer injuries due to forward movement of passengers in rear seats at the moment of frontal collisions, and evaluates the effectiveness of rear passengers’ wearing seat belts in reducing injuries of front seat occupants. Since the occurrence of occupant injuries depends considerably on the crash severity, we proposed to use pseudo-delta V in regression analysis to represent velocity change during a collision when analyzing statistical accident data. As the crash severity can be estimated from pseudo-delta V, it becomes possible to make appropriate estimations even when the crash severity differs in data. The binary model derived from the ordered response model was used to evaluate the influence on the injury level based on pseudo-delta V, belted or unbelted status, gender and age. Occupants in cars with a hood in the case of car-to-car frontal collisions were extracted from the statistical data on accidents in Japan. Among 81,817 cars, where at least one passenger was present, a total of 6847 cars in which all passengers sustained injuries and which had at least one rear seat passenger aboard were analyzed. The number of killed or seriously injured drivers is estimated to decrease by around 25% if rear seat occupants come to wear seat belts. Also, the number of killed or seriously injured passengers in front seats is estimated to decrease by 28% if unbelted rear seat occupants come to wear seat belts. Thus, wearing of seat belts by previously unbelted rear seat passengers is considered effective in reducing not only injuries to the rear seat passengers themselves but also injuries to front seat occupants.

Belt-positioning booster seats and reduction in risk of injury among children in vehicle crashes

Durbin DR, Elliott MR, Winston FK (Children's Hospital of Pennsylvania, Philadelphia, PA) More than 12 states have ratified laws that require booster seats for children older than 4 years of age. Most states have restraint laws that address children through age 4 years. The lack of booster seat effectiveness data may be why stronger child restraint laws have not been passed. The relative effectiveness of belt-positioning booster seats versus seat belts alone in decreasing the risk of injury to children aged 4 to 7 years was examined in a cross-sectional investigation. Children aged 4 to 7 years involved in crashes of insured vehicles in 15 states were evaluated. The data were obtained from insurance claims records and a telephone survey. A probability sample of 3,616 crashes involving 4,243 children, weighted to represent 56,593 children in 48,257 crashes, was obtained between December 1, 1998, and May 31, 2002. Analyses were performed to compute the relative risk of injury for children aged 4 to 7 years who were restrained in belt-positioned booster seats versus seat belts. The primary outcome measure was parent report of clinically significant injuries. Restraint use varied by age. Seat belts were used in 42% of 4-year-olds, 72% of 5-year-olds, and 89% of 6- and 7-year-olds. The belt-positioning booster seats were used in 16% of 4-year-olds, 13% of 5-year-olds, and 4% of 6- and 7-year-olds. Injuries occurred in 1.18% of 4- to 7-year-olds, including 1.95% of those in seat belts and 0.77% of those in belt-positioning booster seats. There were 5 deaths among children in seat belts and none in children in belt-positioning booster seats. The odds of injury, adjusted for child, driver, crash, and vehicle characteristics, were 59% lower for children aged 4 to 7 years who were in belt-positioning booster seats versus those in seat belts (odds ratio 0.41; 95% confidence interval 0.20 to 0.86). Children in belt-positioning booster seats had no injuries to the abdomen, neck/spine/back, or lower extremities. Children in seat belts had injuries to all body regions. Children in seat belts were more likely to be in vehicles driven by young drivers and more likely to be situated in the front seat. Children in belt-positioning booster seats were more likely to be positioned in the rear seats. Belt-positioning booster seats were linked to added safety benefits compared with seat belts for children aged 4 to 7 years, including a decrease in injuries classically linked to improper seat belt fit for children.

Comparison of reporting of seat belt use by police and crash investigators: variation in agreement by injury severity

Purpose: To evaluate agreement between police and trained investigators regarding seat belt use by crash victims, according to injury severity. Methods: We used data from the National Accident Sampling System Crashworthiness Data System (CDS) for front seat occupants, 16 years and older, in crashes during 1993–2000. Crashworthiness Data System investigators determined belt use from vehicle inspection, interviews, and medical record information; their assessment was considered the gold standard for this analysis. Occupant severity of injury was categorized in five levels from no injuries to death. We estimated the sensitivity, specificity, and area under receiver operating characteristic curves for police reports of belt use. Results: Among 48,858 occupants, sensitivity of a police report that a belt was used was 95.8% overall and varied only modestly by injury severity. Specificity of a police report that a belt was not used was 69.1% overall; it was the lowest among the uninjured (53.2%) and greatest among the dead (90.4%). The area under the curve was 0.82 (95% confidence interval 0.82–0.83) overall; this was lowest among those not injured (0.75, 95% confidence interval 0.74–0.76) and increased with injury severity to 0.91 (95% confidence interval 0.90–0.93) among those who died. Conclusion: Police usually classify belted crash victims as belted, regardless of injury severity. But they often classify unbelted survivors as belted when they were not. This misclassification may result in exaggerated estimates of seat belt effectiveness in some studies.

The use of generalized estimating equations in the analysis of motor vehicle crash data

The purpose of this study was to determine if it is necessary to use generalized estimating equations (GEEs) in the analysis of seat belt effectiveness in preventing injuries in motor vehicle crashes. The 1992 Utah crash dataset was used, excluding crash participants where seat belt use was not appropriate ( n=93,633). The model used in the 1996 Report to Congress [Report to congress on benefits of safety belts and motorcycle helmets, based on data from the Crash Outcome Data Evaluation System (CODES). National Center for Statistics and Analysis, NHTSA, Washington, DC, February 1996] was analyzed for all occupants with logistic regression, one level of nesting (occupants within crashes), and two levels of nesting (occupants within vehicles within crashes) to compare the use of GEEs with logistic regression. When using one level of nesting compared to logistic regression, 13 of 16 variance estimates changed more than 10%, and eight of 16 parameter estimates changed more than 10%. In addition, three of the independent variables changed from significant to insignificant ( α=0.05). With the use of two levels of nesting, two of 16 variance estimates and three of 16 parameter estimates changed more than 10% from the variance and parameter estimates in one level of nesting. One of the independent variables changed from insignificant to significant ( α=0.05) in the two levels of nesting model; therefore, only two of the independent variables changed from significant to insignificant when the logistic regression model was compared to the two levels of nesting model. The odds ratio of seat belt effectiveness in preventing injuries was 12% lower when a one-level nested model was used. Based on these results, we stress the need to use a nested model and GEEs when analyzing motor vehicle crash data.

Are seat belt restraints as effective in school age children as in adults? A prospective crash study

Objective: To study effectiveness of seat belts for protecting school age children in road vehicle crashes.Design: Crash examinations by trained investigators.Setting: Ten Canadian university based crash investigation centres.Subjects: 470 children...

Estimating seat belt effectiveness using matched-pair cohort methods

Using US data for 1986–1998 fatal crashes, we employed matched-pair analysis methods to estimate that the relative risk of death among belted compared with unbelted occupants was 0.39 (95% confidence interval (CI) 0.37–0.41). This differs from relative risk estimates of about 0.55 in studies that used crash data collected prior to 1986. Using 1975–1998 data, we examined and rejected three theories that might explain the difference between our estimate and older estimates: (1) differences in the analysis methods; (2) changes related to car model year; (3) changes in crash characteristics over time. A fourth theory, that the introduction of seat belt laws would induce some survivors to claim belt use when they were not restrained, could explain part of the difference in our estimate and older estimates; but even in states without seat belt laws, from 1986 through 1998, the relative risk estimate was 0.45 (95% CI 0.39–0.52). All of the difference between our estimate and older estimates could be explained by some misclassification of seat belt use. Relative risk estimates would move away from 1, toward their true value, if misclassification of both the belted and unbelted decreased over time, or if the degree of misclassification remained constant, as the prevalence of belt use increased. We conclude that estimates of seat belt effects based upon data prior to 1986 may be biased toward 1 by misclassification.

Predicting seat belt use in fatal motor vehicle crashes from observation surveys of belt use

There is a large difference between the rates of observed seat belt use by the general public and belt use by motor vehicle occupants who are fatally injured in crashes. Seat belt use rates of fatally injured occupants, as reported in the Fatality Analysis Reporting System (FARS), are much lower than the use rates found in observation surveys conducted by the states. A series of mathematical models describing the empirical relationship between FARS and observed rates were explored. The initial model was a ‘straw man’ and used two simplifying assumptions: (a) belt users and nonusers are equally likely to be involved in ‘potentially fatal collisions’, and (b) belts are 45% effective in preventing deaths. The model was examined by comparing each state's FARS use rate with the predicted rate. The model did not fit the state data points even when possible biases in the data were controlled. We next examined the assumptions in the model. Changing the seat belt effectiveness parameter provided a reasonable fit, but required an assumption that seat belts are 67% effective in preventing fatalities. The inclusion of a risk coefficient for non-belted occupants also provided a reasonable fit between the model and data. A variable risk model produced the best fit with the data. The major finding was that a model consistent with the data can be obtained by incorporating the assumption that nonusers of seat belts have a higher risk of involvement in potentially fatal collisions than do seat belt users. It was concluded that unbelted occupants are over-represented in fatal collisions for two reasons: (a) because of a greater chance of involvement in potentially fatal collisions in the first place, and (b) because they are not afforded the protection of seat belts when a collision does occur.

Mortality reduction with air bag and seat belt use in head-on passenger car collisions.

To assess the efficacy of occupant protection systems, the authors measured the mortality reduction associated with air bag deployment and seat belt use for drivers involved in head-on passenger car collisions in the United States. They used a matched case-control design of all head-on collisions involving two passenger cars reported to the Fatality Analysis Reporting System in 1992-1997, and driver mortality differences between the paired crash vehicles for air bag deployment and seat belt use were measured with matched-pair odds ratios. Conditional logistic regression was used to adjust for multiple effects. There were 9,859 head-on collisions involving 19,718 passenger cars and drivers. Air bag deployment reduced mortality 63% (crude odds ratio (OR) = 0.37, 95% confidence interval (CI): 0.32, 0.42), while lap-shoulder belt use reduced mortality 72% (OR = 0.28, 95% CI: 0.25, 0.31). In a conditional logistic model that adjusted for vehicle (rollover, weight, age) and driver (age, sex) factors, air bags (OR = 0.71, 95% CI: 0.58, 0.87) and any combination of seat belts (OR = 0.25, 95% CI: 0.22, 0.29) were both associated with reduced mortality. Combined air bag and seat belt use reduced mortality by more than 80% (OR = 0.18, 95% CI: 0.13, 0.25). Thus, this study confirms the independent effect of air bags and seat belts in reducing mortality.

Prevention effectiveness of rollover protective structures--Part I: Strategy evolution.

This is the first of three articles that evaluate the health and economic consequences of the use of rollover protective structures (ROPS) on agricultural tractors. The effectiveness of ROPS delivered through alternative intervention strategies is the subject of the three-part study. This part of the study reviews and assesses the evolution of interventions that are known to prevent injuries incurred as a result of tractor overturns. The method used is historical analysis framed against a prevention effectiveness model used by the U.S. Centers for Disease Control and Prevention. Two intervention strategies were found to be potentially effective in preventing injuries from tractor overturns. These strategies are either to install a ROPS on tractors that lack a ROPS or to replace the tractor with one that has a ROPS already mounted. Other prevention factors include the effectiveness and use of seatbelts and the integrity of the ROPS system during an overturn.

Sample Selection in the Estimation of Air Bag and Seat Belt Effectiveness

Because data are collected for only fatal crashes, it is difficult to accurately measure seat belt and air bag effectiveness. The use of safety devices influences survival rates which in turn determine whether a crash is included in the sample, leading to sample selection bias. We propose a simple solution to the selection problem: limiting the sample to crashes in which someone in a different vehicle dies. Empirically, we find seat belts more effective and air bags to be less effective than previously found. The cost per life saved through seat belts is approximately $30,000, compared to $1.8 million for air bags.