Fatal Crashes among Older Drivers: Decomposition of Rates into Contributing Factors
Ann M. Dellinger1,
Jean A. Langlois1 and
Guohua Li2,3
1 National Center for Injury Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA.
2 Department of Emergency Medicine, Johns Hopkins University School of Medicine, Baltimore, MD.
3 Center for Injury Research and Policy, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD.
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ABSTRACT
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This study selected US drivers aged 55 years or older who were involved in fatal crashes in 1990 and 1995 and explored factors that influenced their fatal crash involvement rate. The fatal crash involvement rate (risk of being involved in a fatal crash) can be thought of as the product of the crash fatality rate (risk of dying given a crash), the crash incidence density (risk of crash), and the exposure prevalence (amount of driving). Fatal crash involvement rates increased with age. The relative contributions of the crash incidence densities and exposure prevalences were greater than that of the crash fatality rates. The decomposition methodology was shown to be a useful method for investigating the potential benefit of crash prevention interventions aimed at different components of the fatal crash involvement rate.
accidents, traffic; age factors; automobile driving; mortality; motor vehicles
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INTRODUCTION
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The growing proportion of older persons in the US population has intensified interest in the safety of the older driver. Today there are approximately 34 million persons aged 65 years or older living in the United States; about 26 million are licensed drivers (1
). Census projections estimate that by the year 2020 there will be 53 million persons over the age of 65 years (2
). At current rates of licensure (75 percent in 1998), we can expect approximately 40 million licensed older drivers by the year 2020 (1
3
).
The concern about older drivers arises from their high motor vehicle-related death and injury rates and their potential risk to others on the road. Adjusting for the number of miles traveled reveals that older drivers experience higher crash death rates than all but the youngest drivers (4
). Each year more than 4,000 drivers aged 65 years or older die in motor vehicle crashes and another 170,000 are nonfatally injured (5
). These numbers are likely to rise with the expected increase in older drivers.
This study selected drivers aged 55 years or older who were involved in the most severe crashes (those in which a fatality occurred) and explored factors that influenced their fatal crash involvement rate (number of fatal crashes/number of drivers per year). Specifically, the number of fatal crashes, the number of total crashes, the number of miles driven, and the number of drivers were used to derive the components of the fatal crash involvement rate and to assess their influence on the rate as age increased. The results were then used to illustrate how this information might help target crash prevention efforts toward the component(s) that exert the strongest influence on the fatal crash involvement rate.
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MATERIALS AND METHODS
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The fatal crash involvement rate can be thought of as the product of the crash fatality rate, the crash incidence density, and the exposure prevalence (figure 1). In other words, the risk of being in a fatal crash equals the risk of dying when a crash occurs, multiplied by the risk of a crash, multiplied by the amount of driving done by the age group.
This decomposition methodology (i.e., decomposing or deconstructing a rate into its component parts) has been described in detail elsewhere, exploring gender differences in death rates from motor vehicle crashes (6
) and bicycling (7
). The death rate in these studies was decomposed into subparts that represent the case fatality rate, the risk of injury (incidence density), and the amount of exposure.
Using ages 5564 as the referent group, we expressed the comparison of fatal crash involvement rates between the referent group and those aged 7584 years as a ratio:
 | (1) |
The ratios of the bs, cs, and ds express their contribution to the fatal crash involvement rate ratio from those aged 5564 years compared with those aged 7584 years in terms of both magnitude and direction. For example, if we use 1995 data (found in tables 1 and 2) to calculate the above values, we get
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TABLE 1. Fatal crash involvement rate, crash fatality rate, crash incidence density, and exposure prevalence by age group and year, United States, 1990 and 1995
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TABLE 2. Comparisons of the contributions of the crash fatality rate, crash incidence density, and exposure prevalence with the change in fatal crash involvement rate by age group and year, United States, 1990 and 1995
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Therefore, the magnitude of the difference in the crash fatality rates (b7584/b5564) was 60 percent in a positive direction, the difference in crash incidence densities (c7584/c5564) was 109 percent in a positive direction, and the difference in the exposure prevalences (d7584/d5564) was 52 percent in a negative direction. Each of these differences contributed to an overall 60 percent higher fatal crash involvement rate (a7584/a5564) when comparing drivers aged 7584 years with drivers aged 5564 years. Said another way, the risk of dying and risk of crash were higher for those aged 7584 years; each component contributed to a positive fatal crash involvement rate ratio. The amount of driving was lower for those aged 7584 years. Therefore, this component drove down their fatal crash involvement rate and, subsequently, the difference in fatal crash involvement rates between those aged 7584 years and those aged 5564 years. This type of comparison can be made between the referent and any age group.
In addition, it is possible to compute the relative contribution (RC) of each component part to the difference in fatal crash involvement (i) rates with the following equation:
 | (2) |
Although alternative equations could be used to quantify the relative contributions of the components, the logarithmic transformation used here has two important properties that support its use. First, it assigns the same relative contribution to a factor regardless of which group acts as the referent, and second, it is not sensitive to the order of groups (i.e., order invariant).
As an example, again using 1995 data (table 1) for drivers aged 5564 and 7584 years, the relative contribution of the difference in crash fatality rates to the difference in fatal crash involvement rates can be expressed as:
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Data for the analyses came from three sources: the Fatality Analysis Reporting System, the General Estimates System, and the Nationwide Personal Transportation Survey. The Fatality Analysis Reporting System, maintained by the National Highway Traffic Safety Administration, is a census of all motor vehicle traffic crashes in the United States that occur on a public road and result in at least one death to an occupant or nonmotorist (e.g., pedestrian) within 30 days of the crash. A driver was included in this study whether or not he/she was the person killed. Fatality Analysis Reporting System data supplied the number of drivers involved in fatal crashes by age and sex for these analyses. The General Estimates System, also maintained by the National Highway Traffic Safety Administration, is a nationally representative probability sample of police-reported crashes of all severities (i.e., property damage only, nonfatal injury, fatal injury). Approximately 45,000 crashes are selected each year from which national estimates are made. The General Estimates System supplied the estimated number of crashes by driver age and sex for these analyses. The Nationwide Personal Transportation Survey, maintained by the Federal Highway Administration, is a large telephone survey of households in the United States. This survey gathers information about the nature and characteristics of personal travel. Estimates of the number of licensed drivers and the number of miles driven by age and sex were obtained from the Nationwide Personal Transportation Survey.
Both the Fatality Analysis Reporting System and the General Estimates System provide data on a yearly basis; the Nationwide Personal Transportation Survey is conducted less often, usually every 57 years. Data from all three sources were available for 1990 and 1995; therefore, we used information from these 2 years in our analyses. However, there were differences in the Nationwide Personal Transportation Survey methodology between 1990 and 1995. These differences, for example, using a written travel diary instead of memory recall, were estimated to have increased the reported number of trips taken and, therefore, miles driven by 22 percent over the real increase in 1995 compared with 1990 (8
). To assess whether differences in survey methodology between the 1990 and 1995 versions of the Nationwide Personal Transportation Survey affected our results, we adjusted the 1990 data by increasing the number of miles driven by 22 percent and reanalyzed. Because the number of miles driven was not used to calculate the fatal crash involvement rate (or the crash fatality rates) directly, these rates and the 1995 incidence density calculations remained unchanged. However, the adjustment decreased the incidence densities for 1990, decreasing the magnitude of the differences between 1990 and 1995. The adjustment also decreased the differences in exposure prevalence between 1990 and 1995. Given these differences and to enhance data comparability, we report results based on the 22 percent adjustment of 1990 data.
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RESULTS
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Fatal crash involvement rates
Fatal crash involvement rates are shown in table 1 and figure 2. Both the magnitude and pattern of rates by age group were nearly identical in 1990 and 1995. As expected, rates increased with age. However, the magnitude of these increases differed. There was little difference in the rates between the two youngest age groups, but rates increased sharply after age 74, with the largest increase occurring in those aged 85 years or older. The fatal crash involvement rate for drivers aged 85 years or older was nearly three times higher than that for all drivers aged 5574 years.

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FIGURE 2. Fatal crash involvement rates for older drivers, United States, 1990 and 1995. Age group is expressed in years.
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Components of the fatal crash involvement rates
Crash fatality rates.
The magnitude and pattern of the crash fatality rates (risk of dying given a crash) by age group were similar in 1990 and 1995 (table 1; figure 3). As with the fatal crash involvement rates, crash fatality rates increased as age increased, with a small increase between those aged 5564 years and those aged 6574 years and a sharp increase after the age of 74 years.

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FIGURE 3. Crash fatality rates for older drivers, United States, 1990 and 1995. Age group is expressed in years.
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Crash incidence density.
Crash incidence density (risk of crash) increased with age, the greatest increase again occurring after the age of 74 years. Although the shape of the curves was similar in 1990 and 1995, the magnitude was greater in 1990, indicating more crashes per miles driven in 1990 for each age group (table 1; figure 4).

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FIGURE 4. Crash incidence density for older drivers, United States, 1990 and 1995. Age group is expressed in years. One mile = 1.61 km.
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Exposure prevalence.
Table 1 and figure 5 show the average annual number of miles driven for persons aged 55 years or older. As age increased, the average number of miles driven declined. The shapes of the curves for 1990 and 1995 are parallel, but each age group drove more miles in 1995.

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FIGURE 5. Exposure prevalence for older drivers, United States, 1990 and 1995. Age group is expressed in years. One mile = 1.61 km.
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Contributions to the differences in fatal crash involvement rates
Using ratios.
Drivers aged 5564 years were compared with three other age groups: drivers aged 6574 years, 7584 years, and those 85 years or older. Table 2 shows these comparisons as ratios of the crash fatality rates, crash incidence densities, and exposure prevalences. This table displays several key findings.
First, with the exception of one comparison between the youngest age groups in 1990, the percentage of change in each of the components followed monotonic trends with age. That is, the percentage of change increased as the age of the comparison group increased for the crash fatality rates and the crash incidence densities and decreased for the exposure prevalences. Second, with only one exception, the percentage of change in crash incidence densities was greater than the percentage of change in either the crash fatality rates or the exposure prevalences. This relation held over time; that is, results were similar for 1990 and 1995 and across age groups. Finally, the percentage of change in the crash incidence densities increased (e.g., from 55 percent to 402 percent in 1990 and from 5 percent to 276 percent in 1995) more than either the crash fatality rates (e.g., from -2 percent to 80 percent in 1990 and from 15 percent to 114 percent in 1995) or the exposure prevalences (e.g., from -32 percent to -69 percent in 1990 and from -26 percent to -65% in 1995) (table 2).
Using natural logarithmic transformations.
The relative contributions of each component part to the difference in fatal crash involvement rates compared with the group aged 5564 years are displayed in table 3. These results show that, in general, the incidence densities and exposure prevalences made the largest relative contributions; for those aged 7584 years, their contributions were nearly equal in 1990 and 1995. The contributions of the incidence densities for the group aged 6574 years differed in 1990 and 1995; otherwise the relative contributions of the incidence densities were very similar in both years. The contributions of the exposure prevalence declined with age, and there was a distinct drop in the size of the relative contribution at age 75 years. With the exception of the group aged 6574 years in 1990, the contribution of the crash fatality rate remained fairly uniform with age.
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TABLE 3. Relative contributions of the crash fatality rate, crash incidence density, and exposure prevalence to the change in fatal crash involvement rate by age group and year, United States, 1990 and 1995
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DISCUSSION
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This study explored the effect of three factors on the fatal crash involvement rate of drivers aged 55 years or older: the crash fatality rate, the crash incidence density, and the exposure prevalence. The crash fatality rate (number of fatal crashes/number of total crashes) remained constant within age groups; however, the crash incidence density and the exposure prevalence displayed notable changes. For each age group, the crash incidence density (the number of crashes per million miles driven) was lower in 1995, while the average number of miles driven was higher. From one perspective, this would indicate an improvement in motor vehicle safety, that is, that during a period of increased travel there were fewer crashes. However, in spite of this improvement in one component of the fatal crash involvement rate during the study period, the rate itself remained unchanged in each age group (table 1; figure 4).
The exposure prevalence (average number of miles driven per driver) increased from 1990 to 1995 for every age group. At the same time, the older the age group of the driver, the lower was their exposure prevalence. The crash incidence density and the exposure prevalence influenced the fatal crash involvement rate in opposite directions; that is, the crash incidence density increased with age and the exposure prevalence decreased.
In sum, the three components of the fatal crash involvement rate behaved quite differently during the study period: The crash fatality rates increased with age but were stable over time (figure 3), the incidence densities increased with age and decreased over time (figure 4), and the exposure prevalences decreased with age and increased over time, ultimately leading to a stable fatal crash involvement rate between 1990 and 1995.
This study also quantified the contribution of each component in two ways, using ratios and using logarithmic transformations. Each method provided different pieces of information. The ratios revealed how the difference in each component contributed to the difference in fatal crash involvement rates when compared with the referent. Using data from 1995 and comparing persons aged 85 years or older with those aged 5564 years, we determined that the 186 percent difference in fatal crash involvement rates was a function of the 114 percent difference in crash fatality rates, the 276 percent difference in incidence densities, and the 65 percent difference in exposure prevalences (table 2). Because it is not possible to simply add positive differences and subtract negative differences to calculate the difference in fatal crash involvement rates, these results are not completely intuitive; therefore, we also assessed the relative contribution of each component as a percentage of the difference. The relative contributions (using logarithmic transformations) quantified the contribution of each component given the contributions of the other two; the contributions of the three components summed to 100 percent by definition. Therefore, using the same example, the 186 percent difference in fatal crash involvement rates was explained by a 24 percent contribution from the crash fatality rate, a 42 percent contribution from the incidence density, and a 33 percent contribution from the exposure prevalence (table 3). Therefore, these results suggest that the crash incidence density and the exposure prevalence exerted more of an influence on the difference in fatal crash involvement rates than the crash fatality rate. In other words, the risk of crashing and the amount of driving explained more of the difference in fatal crash involvement than the risk of dying when there was a crash.
The main purpose of this study was to decompose or deconstruct the fatal crash involvement rate and to assess how the contributions of the components changed with age. This information might then be used to inform prevention efforts. For example, if the crash fatality rate was contributing most to the fatal crash involvement rate, decreasing the likelihood of a fatality once a crash has occurred (e.g., increasing the crashworthiness of vehicles for older persons or improving emergency treatment procedures) would be expected to decrease the fatal crash involvement rate. If the crash incidence density contributed most, efforts aimed at decreasing the number of crashes would be appropriate (e.g., automatic vehicle braking when a crash is imminent). If the exposure prevalence was contributing most, prevention efforts aimed at lowering the number of miles driven (e.g., increased availability of alternative transportation modes) would be expected to decrease the fatal crash involvement rate. This study found that all three components made important contributions (table 2), suggesting that prevention efforts aimed at each component may be warranted.
Many types of interventions have been proposed to decrease the risk of motor vehicle crashes among older drivers: those targeted toward the vehicle, the roadway, and the driver. Interventions based on vehicle design include improvements in safety belts that make them more comfortable and easier to reach and attach, adjustment of seats and pedals with the needs of older persons taken into consideration, regulating the mounting height of headlights to decrease glare during nighttime driving, and using clear and simple instrument panel dials (not reflected on the windshield) to make them easier to read (9
, 10
). These types of interventions could decrease both the probability of a crash and the severity of a crash, which would then affect the crash fatality rate and the crash incidence density.
Suggested roadway improvements have included changes to highway signs that would increase their legibility, conspicuousness, visibility distance (the distance that a driver needs to detect a sign, read it, and take appropriate action), consistency (use of the same signs for the same message), and redundancy (for more advanced notice of upcoming traffic situations). In addition, roadways could be improved by reflectorizing pavement markings, using painted raised channelization (sloping curbed medians) and using more protected left turn operations (9
, 11
). These interventions would be expected to decrease the probability of a crash, affecting both the crash fatality rate and the crash incidence density. Moreover, these improvements would positively impact all drivers.
There have been a variety of suggested interventions targeted at the older driver. There are education and training programs that aim to help drivers identify individual problems so that they can compensate for diminished capabilities and/or to improve their performance (12
14
). There are medical advisory boards operating in several states that make licensing decisions based on medical assessment (15
) and state policies for license renewal that mandate more frequent renewal procedures based on age (these may include vision, knowledge, medical, or road testing) (16
18
). The effectiveness of these policies with respect to older driver crashes is not clear. If found effective, these types of interventions would be expected to affect the number of drivers, the amount of miles driven, the number of crashes, and the number of fatal crashes, in other words, all components of the fatal crash involvement rate.
Alternative transportation has been suggested as a means to decrease crashes by older drivers by providing other acceptable transportation options. These options include better pedestrian facilities (e.g., sidewalks, crossings, appropriate signal timing), public mass transit, paratransit, taxis, and community-based systems that operate at the local level (9
, 19
21
). All of these options would decrease the number of miles driven, reducing the exposure prevalence of users.
Conducting research to assess the effectiveness of these interventions is a challenging next step in the process of increasing motor vehicle safety for older drivers.
Potential limitations exist in our use of the Fatality Analysis Reporting System, General Estimates System, and Nationwide Personal Transportation Survey data sets. There are a small number (about 2 percent) of fatal off-road crashes and crashes in which the fatality occurs after the 30-day cutoff period that are not included in the Fatality Analysis Reporting System. The General Estimates System does not include crashes never reported to the police; however, these are the less severe crashes. Exposure data were derived from the Nationwide Personal Transportation Survey. Because of the changes in survey methodology between 1990 and 1995, adjusting the 1990 data by 22 percent may not have fully compensated for these changes. Moreover, the appropriateness of the adjustment may differ with age. However, the striking consistency of these estimates (figure 5) supports the adequacy of the adjustment.
In addition, the fatal crash involvement rate measures crashes in which someone died. Therefore, fatal crashes in which the fatality was not to the older driver are included. This makes the interpretation of the crash fatality rate more difficult. For instance, the crash fatality rate would not be as good an indicator of the contribution of frailty as the case fatality rate (number deaths/number injuries) would.
Notwithstanding these limitations, this study has yielded important results. Decomposing the fatal crash involvement rate into smaller component parts suggested that there may be gains and losses in motor vehicle safety that are masked by using broader outcome measures (like the fatal crash involvement rate). In this study, a decrease in the crash incidence density (a gain in safety) would not have been apparent if only evaluating the fatal crash involvement rates for 1990 and 1995. Moreover, this potential gain in safety was offset by increases in the exposure prevalence.
Decomposition methodology is an exploratory data analysis tool that has been shown to be a useful method for investigating the contributions of the various components of the fatal crash involvement rates of drivers over the age of 55 years. Once the contributions of these elements are understood and verified in real-world settings, more targeted interventions can be planned and implemented. It may also be useful to consider decomposing the fatal crash involvement rate in other age groups to determine how the relative contributions of different components influence the fatal crash risk at different ages. This methodology could also be used to investigate other types of crash situations, other demographic factors related to crashes, and other injury problems when exposure data are available.
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NOTES
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Correspondence to Dr. Ann M. Dellinger, National Center for Injury Prevention and Control, Centers for Disease Control and Prevention, 4770 Buford Highway NE, Mailstop K-63, Atlanta, GA 30341 (e-mail: adellinger{at}cdc.gov).
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Received for publication March 7, 2001.
Accepted for publication July 3, 2001.