|NHTSA Report Number DOT HS 806 890||February 1986|
Charles J. Kahane, Ph.D.
The purpose of child safety seats is to reduce the number of child passengers killed or injured in motor vehicle crashes. The seats function by absorbing and safely distributing crash impact loads over the child's body while holding the child in place and preventing contacts with vehicle interior components or ejection from the vehicle. Seats have to be convenient and easy to use.
Federal Motor Vehicle Safety Standard 213 specifies performance and labeling requirements for child safety seats. The objectives of this agency staff evaluation are to measure the effectiveness, benefits and usage of safety seats and other safety measures for child passengers aged 0-4. The study is based on statistical analyses of National Accident Sampling System, Fatal Accident Reporting System and State accident data, analyses of sled test and compliance test results, and observational surveys of restraint system usage and misuse. It was estimated that:
Safety seats for infants and small children riding in motor vehicles are one of the most successful auto safety innovations of the 1960's. They are designed to hold children in place during a crash and prevent them from being thrown into the instrument panel or other parts of the vehicle or from being ejected from the passenger compartment. Moreover, they are specifically tailored to a child's anatomy and designed to restrain a child without applying dangerous forces to vulnerable body regions. By contrast, the lap and shoulder belts that come with the vehicle are designed for adults and are in several ways inappropriate for small children.
At first, the seats were purchased only by a minority consisting of the most safety-conscious parents. During the 1970's, a massive educational campaign by the medical community, consumer groups. safety seat manufacturers and insurance companies, among others, made a much wider public aware that children needed safety seats. Between 1978 and 1985 every State, beginning with Tennessee, passed laws requiring safety seats for young child passengers. The public has supported the laws and generally understands why they are needed. By 1984, close to half of the child passenger population aged 0-4 was rid1ng in safety seats.
The National Highway Traffic Safety Administration has long had a critical role in child passenger safety. Federal Motor Vehicle Safety Standard 213 which took effect on April 1, 1971, required that any child seat marketed for use in a vehicle be designed to restrain and protect children in a crash: it had to be attachable within a car by the car's belt system and it would have to distribute rather than concentrate crash forces over the child's torso. A new version of Standard 213 took effect on January 1, 1981, with a 30 mph dynamic test requirement. In the dynamic test, dummies' excursion beyond the confines of the seat had to be within specified limits. So did head and chest forces. The NHTSA standards helped eliminate nonsafety or inadequate seat designs from the market.
In addition to promulgating the standards, NHTSA held conferences and workshops on child passenger protection throughout the United States, provided information and resources to the State and local groups seeking to increase usage of safety seats and encouraged States to fund child passenger safety programs under Section 402 of the Highway Safety Act of 1966.
Executive Order 12291 (February 1981) requires agencies to evaluate their existing major programs, including any program whose annual effect on the economy is $100 million or more. The objectives of an evaluation are to determine the actual benefits--lives saved, injuries prevented, damage avoided--and costs of safety devices produced and sold in response to agency standards or programs and to assess cost~effectiveness.
This summary report contains the principal findings and conclusions of NHTSA's evaluation of what has been accomplished to enhance the safety of children aged 0-4 who are passengers in motor vehicles. The report provides estimates of the number of children actually being saved by safety seats each year. The growth in that number measures the success of the child passenger safety program. The most important parameter for calculating benefits is an estimate of the effectiveness of safety seats in actual use: the average reduction of casualty risk for children in safety seats (including correctly used and misused seats) relative to unrestrained children.
The exact effectiveness of safety seats (in actual use) is still not agreed upon by the safety community and a wide variety of estimates ranging as high as 90 percent is quoted in the literature. The evaluation's primary objective was to pin down an in-use effectiveness estimate, but in the process it was found that the goal is a moving target. Effectiveness is not constant, but has increased year by year as an ever greater percent age of the safety seats in use are being used correctly.
That brings up the second goal of the evaluation: a more complete understanding of the problem of improperly used seats. It is well known that an alarming percentage of safety seats (65 percent in one study) are not being used according to manufacturers instructions; it is generally believed that misuse of seats is the major factor holding down effectiveness and benefits. But it has to be recognized that some types of misuse are far more detrimental than others. The evaluation identifies the more common use modes for each major type of safety seat and then groups them into three categories:
Correct use - exactly as recommended by the manufacturer or close enough that there would not be a significant loss of safety benefits.
Partial misuse - significantly lower effectiveness than correct use, but there should still be substantial benefits if the crash is not too severe. Something is holding the child within the seat and something is anchoring the seat within the vehicle. But the child will experience more excursion or crash forces and/or the seat will be more likely to fail, because of the way it is misused (e.g., not using the required tether, misrouting the lap belt).
Gross misuse - situations where children would be thrown from the seats or the seats (with children in them) would become projectiles in a crash--basically like an unrestrained condition. (Also included in this category were children riding in feeder seats, infant carriers, or other devices intended for use in the home, not the car. By 1984, only 0.3 percent of child passengers were in such devices, although they were much more common in the 1970's. They could not be separated from grossly misused safety seats because the accident data, as well as many of the observational surveys, likewise do not identify them as a distinct category but merely include them among "safety seat users.")
The evaluation estimates the frequencies of the three categories, year-by-year, and the average effectiveness of each category. That makes it possible to estimate overall effectiveness (the weighted average of the three categories) and lives saved, year-by-year. The difference in benefits between 100 percent correct usage and the actual mix of correct use and misuse is the bottom-line effect of the problem of misused seats.
In addition, the evaluation tracks the overall usage of safety seats, year-by-year. It gives a preliminary comparison of the effectiveness of the major types of seats--when correctly used and, more importantly, when their frequency of misuse is taken into consideration. It estimates the effectiveness of two other child passenger safety measures that should be employed only when a certified safety seat is not available: restraining a child with an adult lap belt only or having the child ride unrestrained in the back seat. It also estimates the benefits of moving a restrained child from the front to the back seat.
The evaluation is based on analyses of accident data, observational surveys of restraint system usage and sled tests with restrained and unrestrained dummies.
Accident analyses have been per formed in anticipation of this study since 1978. But the most recent data have been the most meaningful because they contain much larger samples of safety seat users. NHTSA's Fatal Accident Reporting System provided a good estimate of overall fatality reduction. The agency's In-depth accident data based on probability sampling--the National Accident Sampling System (NASS) , National Crash Severity Study (NCSS) and Restraint Systems Evaluation Project (RSEP)--were combined to obtain an estimate of serious injury reduction. Pennsylvania data for 1981-83 were used for calculating Injury-reducing effectiveness, overall and by injury type. State data from New York, Maryland, New Jersey and Idaho were analyzed for this evaluation, while published studies of Tennessee, Michigan and Washington data were reviewed. The accident data analyses, even though they are the basis for this study's overall effectiveness estimate, nevertheless have three shortcomings. They do not distinguish between correctly used and misused seats; the estimate derived from any data file is valid, at best, only for the year in which the data were collected--in later years, when a larger percentage of the seats would have been used correctly, effectiveness would have risen; the data are themselves biased because the investigators (police, NHTSA contractors) tended to report certain safety scat users, especially the gross misusers, as "unrestrained." A unique study performed in North Carolina during 1983-1984, however, compared police-reported safety seat use to actual use, by misuse mode (based on detailed interviews in which parents explained how they used each component of the safety seat)--thereby making it possible to correct for the biases in the other studies.
The comparison of correctly used and misused seats was based primarily on a sled test project conducted especially for this evaluation. The project differed from earlier sled test studies with child dummies in that:
The data from this special study were complemented by a statistical analysis of 1981-84 compliance test results for Standard 213--frontal sled tests of correctly used and partially misused safety seats. The compliance tests provided data on a variety of safety seat models which were not included in the special study. They employed a more severe deceleration pulse than the tests in the special study; as a result, the seat types which performed best in the compliance tests were not the same as the best performers in the special study--although, in both test series, all correctly used seats performed very well relative to misused seats or unrestrained dummies.
The sled test results were used to obtain effectiveness estimates for safety seats, correctly used and in each of the misuse modes that commonly occur in actual practice. Next, observational surveys of safety seat usage indicated the relative frequency of occurrence of each seat type/misuse mode combination. The effectiveness estimates were then averaged (weighted by frequency of occurrence) to obtain an overall estimate of serious injury reduction for the mix of correctly used and misused seats that was actually found in the traffic population. Since that mix changed from year to year, so did the overall estimate.
The most detailed observational survey of safety seat usage was conducted at Hardee's restaurants during 1984. The make/model of safety seat and the exact way in which it was used was recorded for over 1000 children; based on the taxonomy of this evaluation, the data were grouped to estimate the frequency of occurrence of each seat type/misuse mode in 1984. Five other observational surveys gave accurate estimates of overall usage during 1974-84 and (with some interpretation) a split between correct use/partial misuse, on the one hand, and gross misuse, on the other. The Hardee's data, sales trends for safety seats and three parking lot surveys of unoccupied seats made possible a further split between correct users vs. partial misusers. Thus, the frequency of correct users, partial misusers and gross misusers could be estimated year-by- year from 1979 to 1984 and employed for weighting the sled test results to obtain year-by-year estimates of overall effectiveness and benefits.
Finally, these year-by-year effectiveness estimates from the sled tests/usage surveys were compared to the police-reported accident data analyses (which were corrected for the usage reporting biases found in the North Carolina study). The agreement was almost perfect: effectiveness (in actual practice) was just below 30 percent in the studies based on pre-1979 accident data and just over 45 percent by 1984. Moreover, the sled tests accurately estimated safety seat effectiveness in NASS (57 percent, since gross misusers are counted as "unrestrained") and the injury reductions in the various accident studies for lap belt only and for moving an unrestrained child to the back seat. The excellent correlation of the sled test predictions with the results of the accident analyses and the consistent trend among the accident studies themselves (after the year of the data collection and the source of the reporting biases are taken into account) provide an especially high degree of confidence in the overall effectiveness estimates of this evaluation and the year-to-year trend of rising effectiveness. Each of the data sources used in the evaluation had some shortcomings (documented in the text); nevertheless they fit together exceptionally well and the whole picture became clear after assembling the parts.
The sled test data analyzed in this evaluation showed that each of the major types of approved safety seats currently on the market is highly effective when correctly used. They do not support a conclusion that any particular type of seat (correctly used) is significantly more effective than the other types (correctly used) over the full range of frontal crash types that occur on the highway--although the tests did show that certain types of seats may excel in some specific crash situations.
Some topics were not addressed in this evaluation and remain to be resolved in follow-up studies: the effectiveness of correctly used and misused toddler seats in side impacts, by seat position--to be studied using sled tests supported by accident data; the effectiveness of correctly used vs. misused infant seats; booster seats vs. adult belts for children age 5 or older; the compatibility of safety seat designs with the various types of safety belt systems that are installed in passenger vehicles; a State-by-State analysis of safety seat usage vs. the type of buckle-up law, the level of enforcement, and the States' educational and promotional activities in child passenger safety--to identify the combinations of factors that best increase usage of safety seats.
The principal findings and conclusions of this evaluation are the following:
|Lives Saved in:||1979||1980||1981||1982||1983||1984|
|By safety seats||30||47||60||88||135||158|
|By lap belts||8||9||10||15||24||34|
|Hospitalizations Prevented||Children Avoiding any Injury|
|By safety seats||1,020||17,000|
|By lap belts||330||4,000|
|Percent of Children in||1974||1979||1980||1981||1982||1983||1984|
|Lap belt only||4||3||4||4||6||9||14|
|Child seats or lap belt||20||18||24||28||38||51||60|
|Number of States with buckle-up laws in effect at the end of the year||0||1||2||3||13||31||46|
|Percent of Child Seats in Use||1974||1979||1980||1981||1982||1983||1984|
|Grossly misused safety seats/home child carriers used as car seats||61||50||42||37||30||24||21|
|Percent of All Child Passengers in||1979||1980||1981||1982||1983||1984|
|Correctly used safety seats||3||4||5||8||13||18|
|Partially misused seats||5||8||10||14||19||18|
|Grossly misused safety seats/home child carriers used as car seats||7||8||9||10||10||10|
|(Not in a child seat)||85||80||76||68||58||54|
|Age of Child||Percent Using Safety Seats||Number of States in 1985 Requiring Safety Seat that Age|
|Restraint System Use in North Carolina (%)|
|Age of Child||Safety Seats||Lap Belt Only||Safety Seat or Lap Belt|
(The North Carolina buckle-up law applies to children under 2, requiring a safety seat for infants under 1 and a choice of seat or belt for 1 year olds.)
|Percentage Reduction of||Fatalities||Hospitalizations||Nonserious Injuries|
|Lap belt only||33||50||30|
|Unrestrained: back seat vs. front seat||27||27||25|
|Safety seat users: back seat vs. front seat||20||20||20|
|Reduction in Fatalities/Hospitalizations||1979||1980||1981||1982||1983||1984|
|Lap belt only (fatality reduction)||33||33||33||33||33||33|
|Unrestrained: back seat vs. front seat||27||27||27||27||27||27|
|Safety seat users: back seat vs. front seat||23||23||22||22||21||20|
|Reduction (%) of Hospitalizations Relative to Front-Seat Unrestrained|
|Correctly used seat||69|
|OVERALL (1984 mix of correct/misused)||48|
|Reduction (%) of Hospitalization Relative to|
|Front-Seat Unrestrained||Back-Seat Unrestrained||Front-Seat Restrained*|
|Correctly used seat||73||63||11|
|OVERALL (1984 mix of correct/misused)||58||43||20|
*I.e., correctly used: back vs. front; partially misused: back vs. front; etc.
|Fatality Reduction, 1980-84 (%)|
|Infants (age less than 1)||43|
|Toddlers (age 1-3)||44|
|Average of both groups||43|
Each of these numbers would be about 3 percent higher for 1984, alone, since a larger proportion of the seats was used correctly than in 1980-83.
|Percent Reduction by Body Region||Pennsylvania 1981-1983 Moderate Injuries* All Crashes||Sled Tests, 1984 Mix Hospitalizations Frontal Crashes|
*Police-reported levels K, A or B.
|Type of Seat||Example of a Best-Selling Make/Model||Share of 1984 On-the-Road Mix||Correct Use||Partial Misuse||Gross Misuse|
|Tethered (belt thru frame)||Strolee Wee Care 197, 599||17||9||58||33|
|Tethered belt-around||GM/Century Child Love Seat||3||18||79||3|
|Tetherless belt-around||Bobby Mac Champion||9||12||74||14|
|Tetherless, harness only||Century 100||18||53||21||26|
|Tetherless, partial shield||Questor One-Step||20||56||29||15|
|Tetherless, full shield||Cosco/Peterson Safe-T-Shield||2||76||24||0|
|Booster (using car's shoulder belt or tether-harness)||Kolcraft Tot Rider XL||12||40||45||15|
|Infant belt-around||GM/Century Infant Love Seat||10||41||48||11|
|Infant (belt thru frame)||Most convertible seats, when used by infants||5||45||45||10|
|TOTAL OR AVERAGE||100||39||40||21|
|Type of Seat||Reduction (%) of Hospitalizations Based on Frontal Sled Tests|
|Tethered (belt thru frame)||34|
|Tetherless, harness only||45|
|Tetherless, partial only||51|
|Tetherless, full shield||62|
|Booster (using shoulder belt or tether-harness)||54|
|Infant seats (both types)||43*|
|*Fatality reduction based on 1980-1984 accident data|
|Misuse Mode||Percent of All Partial Misusers|
|Lap belt misrouted thru frame||23|
|Tether not used||21|
|Harness not used (lap belt correctly routed around child)||20|
|Booster seat - no shoulder belt/tether harness||13|
|Infant seat - - facing wrong way||7|
|Bobby Mac - shield not used, else correct||5|
|Tether not used and belt misrouted||4|
|Misuse Mode||Percent of All Gross Misusers|
|Child not secured in seat||37|
|Child not secured and seat not anchored in car||33|
|Seat no anchored in car||27|
|Type of Seats||Individual Item Misused||Percent of Seats of those Types|
|All seats with tethers||Tether not used||85|
|Booster seats||Shoulder belt/tether not used||60|
|All seats with plain harness||Harness not used||36|
|Lap belt through frame||Lap belt routed too low||24||}35|
|Lap belt not used at all||11|
|Infant seats||Seat facing wrong way||33|
|Lap belt around child||Lap belt not used||11|
|Seats with full shields||Shield not used||9|
|Seats with integral harness/partial shield||Harness not used||8|
NOTE: The identification of partial vs. gross misuse takes into account simultaneously the status of each of the seat's hardware items and the design of the seat. It cannot be derived from the percentages shown in the above table.
|Type of Seat||Partial Misuse Mode||Effectiveness (Percent)|
|Tethered||Tether not used-otherwise OK||49|
|Tethered||Tether not used and lap belt too low||44|
|Tetherless - harness only||Lap belt too low||46|
|Booster||No shoulder belt/tether harness||59*|
*However, in the 1981-84 NHTSA compliance tests, which used a "harder" crash pulse, the booster seat with no shoulder belt/tether harness had significantly more severe head injury predictions than the tethered seat with the tether not used. (The other two misuse modes were not tested.)
|Overall Usage||Actual Effectiveness
(1984 Correct/Misuse Mix)
(All Seats Used Correctly)
|1984 level for infants (no dropoff for older children)||233||360|
|100 percent usage||341||527|