Current Trends Comparison of Observed and Self-Reported Seat Belt Use Rates -- United States

To measure compliance with seat belt use laws, most states have estimated belt use by direct observation of vehicle occupants. In addition, since 1984, several states have recorded seat belt use data as part of the Behavioral Risk Factor Surveillance System (BRFSS) telephone survey (1-3). Previous studies indicate that telephone surveys usually report higher belt use than do observation surveys conducted in similar areas at similar times (4,5). A systematic comparison of self-reported belt use rates in 15 states* from the 1987 BRFSS with observed belt use rates in 1987 in the same states follows.

The BRFSS telephone surveys used similar designs in each state. A statistically valid random sample of all adults in each state was obtained by random digit dialing. Each survey asked the same questions and classified the responses into the same five categories. Thus, the BRFSS surveys in each state can be considered replications of the same survey.

For the observation surveys, some states used probability sampling techniques to select locations and times. These surveys produced statistically valid estimates of the actual belt use rates under the conditions surveyed.** Other states used locations and times selected by judgment. The accuracy of the estimates from these surveys is unknown.

In the BRFSS self-reported surveys, the number of affirmative answers was derived in two ways: as the total number of respondents who reported "always" using seat belts and as the sum of those who reported "always" and "nearly always" using them. The average self-reported "always" use exceeded observed use by about 8% and ranged from 11% below observed use to 24% above. The average "always or nearly always" self-reported use exceeded observed use by 27%, with a range of 12% above observed use to 39% above. To further examine the relationship between observed and reported seat belt use, simple linear regressions were used for each state (Figures 1 and 2). The relation is described moderately well by either regression; approximately 54% of the variation in prevalence of observed use was accounted for by the prevalence of self-reported use. In the regression line for which "always" was used as the definition, a 1 percentage point increase in self-reported use accounted for a 0.7 percentage point increase in observed use. When "always" and "nearly always" were used, a 1 percentage point increase in self-reported belt use accounted for a nearly 1 percentage point increase in observed use. However, these figures are valid only within the range of the self-reported seat belt use data. Reported by: Office of Driver and Pedestrian Research, National Highway Traffic Safety Administration. Div of Nutrition, Center for Health Promotion and Education, CDC.

Editorial Note

Editorial Note: Worldwide experience has demonstrated that seat belt use laws can substantially reduce deaths and injuries on highways. More than 30 foreign countries, 31 states, and the District of Columbia now have laws requiring adult drivers and passengers to use seat belts.

Direct observation surveys of seat belt use, if properly designed and conducted, can produce accurate estimates of use. However, observation surveys are expensive to conduct and usually observe only shoulder belt use. Furthermore, although they can estimate a driver's or occupant's age and sex, they cannot gather other information useful in understanding belt use, such as trip purpose or attitudes about belt use laws. Telephone surveys provide the opportunity to collect these additional data. They may also be less expensive to design and conduct than observation surveys. However, telephone surveys can record only the respondents' stated behavior, not their actual behavior.

Some of the divergence in the data analyzed here may be due to the fact that the self-reported data were collected each month throughout 1987 and thus estimate average belt use throughout the year. The observed data were at collected different times in each state. Furthermore, the self-reported data were drawn from a sample of the entire state while observed data from some states came from only a few sites. The moderate fit of the regression lines means that they are useful in describing general relations between observed and self-reported belt use, but they should not be used to predict observed use in a single state when only one self-reported survey is available.

More studies such as these are needed to establish reliably the relationship between the results from observation surveys and BRFSS telephone surveys. The results from observation surveys could then be used to help interpret the BRFSS responses and translate them into approximate actual belt use levels. The BRFSS data in turn could be used to investigate characteristics of belt users and nonusers that cannot be determined from observation surveys and to provide information on temporal trends without the expense of observation surveys. In these ways, the usefulness of both types of surveys would be enhanced.

References

1. CDC. Behavioral risk-factor surveillance--selected states, 1984. MMWR 1986;35:253-4.

2. CDC. Behavioral risk-factor surveillance in selected states--1985. MMWR 1986;35:441-4.

3. CDC. Behavioral risk factor surveillance--selected states, 1986. MMWR 1987;36:252-4.

4. Hunter WW, Stutts JC, Stewart JR, Rodgman EA. Overrepresentation of seat belt nonusers in traffic crashes. Chapel Hill, North Carolina: University of North Carolina, Highway Research Center, 1988.

5. Wagenaar AC, Streff FM, Molnar LJ, Businski KL, Schultz RH. Factors related to nonuse of seat belts in Michigan. Washington, DC: US Department of Transportation, 1987; DOT publication no. (HS)807-217. *California, Florida, Hawaii, Illinois, Indiana, Maryland, Minnesota, Missouri, New Mexico, New York, North Carolina, Ohio, Tennessee, Utah, and Washington. **Most surveys took place during daylight hours and measured belt use by the driver and right front seat passenger.

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