Predictors of safety belt use among crash-involved drivers and front seat passengers: adjusting for over-reporting

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Abstract

Police-reported crash data are rarely used to investigate safety belt use and its predictors, even though these data have a number of advantages over data collected in roadside surveys. It has been widely recognized that motorists tend to over-report their safety belt use to police when mandatory belt use becomes law. In this paper, we use a logistic regression model that allows for misclassification errors in outcome variable to examine predictors of safety belt use among crash-involved drivers and front seat passengers. Our analysis shows significant associations between occupant characteristics, driving circumstances, and safety belt use. Alcohol involvement has the strongest negative association with safety belt use, but this association would be considerably underestimated without adjusting for the over-reporting of safety belt use in police-reported crash data. The adjusted belt use rate among front seat occupants with at least nonincapacitating injuries is about 81%, compared to 90% in police-reported crash data.

Introduction

While it is a truism that ‘safety belts save lives’ and most states have mandatory safety belt use laws in the United States, many people still do not use safety belts. It had been projected that safety belts saved about 4573 lives in 1988 when the use rate was 45% and could have saved about 11 101 lives if the use rate was 80% in the United States (Partyka and Womble, 1989). As of December 1995, the belt use rate varied with states from 40 to 86% with a national average estimated at 68% (National Highway Traffic Safety Administration, 1996). Hawaii has consistently had a use rate above 80% and currently is ranked the sixth highest of all states with data available. Primary enforcement which allows police to stop vehicles for failure to wear a safety belt is believed to have been an important factor in achieving a high use rate (Kim, 1991, Escobedo et al., 1992).

There have been many studies estimating safety belt use rates or identifying motorists most likely to be in compliance with mandatory safety belt use laws. These studies typically use specifically designed data collection procedures, involving roadside observational surveys and posterior interviews with vehicle owners identified by license plates or other means. Police-reported crash data are generally not used because of concerns with the accuracy of police-reported safety belt use (Streff & Wagenaar, 1989, O’Day, 1993, Stewart, 1993, Reinfurt et al., 1996). Because of mandatory belt use laws, and the associated fines and other sanctions, motorists have incentives to be less than truthful to law enforcement officials. Stewart (1993) found that among drivers observed in a survey who classified themselves as ‘rarely or never belted’, about 80% were recorded as belted in the police crash reports. In our own analysis using Hawaii data, we have found that among patients admitted to hospitals following motor vehicle crashes, about 25% of those who reported to police that they were belted, reported to doctors or medical personnel that they were unbelted. Blincoe (1994) reported that the National Accident Sampling System (a representative sample of nonfatal motor vehicle crashes in the United States) generally yield higher use rate than observational surveys. Belt use rates among crash-involved and injured motorists could even exceed that among general motorists found in roadside surveys. This, naively interpreted, would suggest that crash-involved motorists are more likely to be belted and using a safety belt increases the risk of injury, which is contradictory to our common wisdom.

Although roadside observational surveys can yield accurate estimates of safety belt use, they have several limitations. First, only limited information about motorist characteristics and belt use can be collected. It is often necessary to make extra efforts such as matching observed motorists to vehicle registration files by license plates and making follow-up interviews with registered owners in order to gather more detailed information. This process often produces a great number of cases with much incomplete data. The response rate by motorists, especially nonusers of safety belts, can be low. Second, the choice of times and locations for conducting observations often restricts the implementation of the survey. It is easier to conduct an observational survey during daytime on weekdays and at sites with high traffic volume, but it may be also less likely to observe nonusers of safety belts who are at higher risk of crashes during daytime or at these locations. Finally, safety belt use is affected by not just motorist characteristics, also by driving circumstances such as weather, time and location. Roadside surveys often preclude the examination of the effects of situational factors on safety belt use.

Despite the lack of accuracy, police-reported crash data are readily available and may be the only source of population-based data with detailed occupant characteristics. In this paper, we examine the relationship between personal characteristics of crash-involved motor vehicle occupants, driving circumstances, and the use of safety belt using police-reported crash data in Hawaii. Elsewhere, it has been suggested that nonusers of safety belts are over-represented in crash- or conviction-involved populations (Reinfurt et al., 1990, Hunter et al., 1993). We will examine whether the associations between personal characteristics, driving circumstances, and safety belt use found in previous observational studies also exist among crash-involved motorists. Of particular interest is the association between alcohol involvement and safety belt use. Past studies have suggested a negative association between alcohol consumption and safety belt use. Reinfurt et al. (1996) found a significant and negative association between alcohol consumption in the past 12 months and safety belt use, but failed to find a statistically significant association between driving under the influence of alcohol and not wearing safety belts. It is understandable that motorists who have driven under the influence of alcohol may be less likely to admit that they did so when interviewed afterwards.

Because police-reported data on safety belt use can be erroneous mainly due to the tendency of motorists to falsely report having worn safety belts, we introduce a logistic regression model that accommodates this type of error in reporting the outcome. With this model, we will assess the degree of bias in the associations between motorist characteristics, driving circumstances and safety belt use induced by this error. We will also estimate the magnitude of the over-reporting of safety belt use in police-reported crash data in Hawaii.

Section snippets

Statistical model

Logistic regression models are commonly used to analyze dichotomous outcome data. However, the event of interest may be misclassified in practice due to various sources of errors such as the inaccuracy of diagnostic test or systematic errors in recording the event of interest. Conventional logistic regression models can be extended to accommodate this type of errors in the outcome variable by introducing the sensitivity and specificity of classifying the event of interest.

Let Yt be a

Results

The results of two analyses are presented. First, we include all drivers and front seat passengers of passenger vehicles involved in crashes, regardless of injury severity. Because most of these motorists were not injured and many of them were ordinary motorists not responsible for causing the crashes, they should be representative of the general driving population to some degree and their actual safety belt use rate should be close to that observed in roadside surveys. In this analysis, we

Discussion

Even though safety belt use has risen substantially in the United States because of the mandatory belt use laws, it has been difficult to further increase safety belt use. In Hawaii, where there is a primary safety belt use law, the use rate has remained around 85% and significant increases beyond this level have not occurred (Kim et al., 1995a). In this paper, we have examined the relationships between the personal characteristics of drivers and front seat passengers, driving circumstances and

Acknowledgements

This research was funded by the National Highway Traffic Safety Administration, US Department of Transportation, Cooperative Agreement No. DTNH22-92-Y-37329, known as the Hawaii CODES Project. We thank the Injury Prevention and Control Program, State Department of Health, the Pacific Basin Rehabilitation Research and Training Center for making the Injury in Hawaii Study data available. The support of the Department of Transportation, State of Hawaii is acknowledge.

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