Older drivers and risk to other road users

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Abstract

This paper examines the degree to which older drivers impose an “excess” risk of death or injury serious enough to require hospitalization on other road users; that is, the amount of risk older drivers impose on others above and beyond the amount imposed by drivers who are not yet old. A data set linking crash information from police accident reports to hospital discharge data in Wisconsin, 1991, was used to analyze excess risk associated with older drivers in two ways. First, the difference in the rate of serious injuries to other road users per 100 million driver age-group miles was used to estimate the total number of serious injuries resulting from the excess risk imposed on others by older drivers. Second, statistical models were used to infer the association between driver age and crash severity while conditioning on a variety of crash-specific information. Drivers aged 65–74 did not appear to impose excess risk of either deaths or injuries requiring hospitalization in either the aggregate or individual level analyses. Drivers aged 75 and over are associated with increased injuries to others, although the actual number is very small; the individual crash-level analysis suggests that a non-trivial part of the excess risk found in the aggregate analysis is a product of confounding.

Introduction

Over the coming decades, the demographics of the driving population will change dramatically because of the aging of the baby-boom generation. It has been estimated that by the year 2000, one-third of all drivers will be aged 55 or over (American Association of Retired Persons, 1992); by the year 2030, older drivers are expected to increase by two-thirds over their current level (Transportation Research Board, 1994). These demographic shifts might be expected to result in a fundamental change in the riskiness associated with driving: it is well known that the number of motor vehicle crashes per unit of distance driven is “U”-shaped, with crash risk increasing slightly between the ages of 55 and 60, but greater increases in risk with each successive five-year interval (Insurance Institute for Highway Safety, 1992). Similarly, the risk of involvement in crashes resulting in a fatality is also “U”-shaped relative to driver age (Insurance Institute for Highway Safety, 1992).

The prospect of more older drivers on the road has generated concern among traffic safety experts and policy-makers alike. One result was that the Transportation Research Board of the National Research Council convened a meeting in 1993 to “address the means by which licensing agencies can discharge their obligation to protect the public from those elderly drivers who cannot operate safely while assuring the mobility of those whose ability to drive safely is undiminished” (emphasis added; Transportation Research Board, 1994). Despite the growing relevance of this issue, research on risks to other road users is at best rare. Evans (1991)analyzed Fatal Accident Reporting System (FARS) data on the age of drivers involved in fatal motor vehicle–pedestrian crashes; a literature search found no other work on risks to other road users. Older-driver research has tended to focus on areas such as the total or personal risk associated with age groups (see, e.g., Insurance Institute for Highway Safety, 1992; Stutts and Martell, 1992); medical interventions (see, e.g., Leveille et al., 1994; Ray et al., 1992); medical conditions (see, e.g., Gresset and Meyer, 1994; Koepsell et al., 1994); and subjective awareness of risk (see, e.g., Holland, 1993; Guppy, 1993).

While the total level of risk faced by older drivers (and their passengers) is an interesting issue in its own right, it is not the critical issue in assessing the likely change in risk to road users in general from an increase in the number of older drivers over the coming decades. The central issue in assessing the effect of more older drivers on road safety in general is the degree to which older drivers impose a higher level of risk on other road users. For example, if older drivers were, say, ten times as likely to be involved in a crash as other drivers, but all older driver crashes were single-vehicle crashes, an increase in the number of older drivers would not affect the risk associated with road use by non-older drivers, even though the number of crashes involving older drivers would increase. If an increase in the number of older drivers is to result in an increased level of risk to road users in general, it must be the case that older drivers impose a greater risk on other road users than not-yet-older drivers do.

This paper uses two methods to examine the question of whether older drivers impose more risk on other road users than do drivers under age 65. The first approach focusses on the difference in risk to others per unit of distance driven by an age group. The resulting estimate of risk differences is then used to calculate the actual number of injuries attributable to the `extra' risk older drivers impose on other road users over and above the level imposed by drivers under 65. By focussing on age groups in the aggregate, this approach ignores the details of specific crashes. The second approach analyzes crash-level data to take account of available information relevant to crash outcomes, such as the types of vehicles involved in a crash. This approach results in estimates of the degree to which driver age is associated with crash severity (as defined by crash outcomes).

Section snippets

Data

The data used in this paper are from the Wisconsin Crash Outcomes Data Evaluation System (CODES) for the year 1991. These data are the result of a matching process linking police accident reports from the Wisconsin Department of Transportation to inpatient discharge records for acute-care hospitals from the Office of the Commissioner of Insurance.1 Because personal identifiers are not available, the data

Methods and results

This section analyzes the Wisconsin CODES data to estimate the level of risk older drivers impose on other road users, and whether this level of risk differs from that imposed by other drivers, i.e. those who are not yet old. Other road users are defined as persons outside the driver's vehicle; while not a perfect measure, this definition distinguishes between persons with some choice about whether to accept the risks associated with a particular person's driving behaviors (i.e., persons

Discussion

Physical skills used while driving (e.g., vision, hearing, reflexes, etc.) generally erode with age. If the erosion of these skills reduces the ability of older persons to drive safely, we would expect not only that they would incur more crashes, but also that as a consequence of these crashes, we would observe more serious injuries. In particular, if the erosion of skills among older drivers makes them more dangerous to other road users, we would expect that persons colliding with older

Conclusion

Data linking police accident reports to hospital discharge data covering all reported crashes in Wisconsin in 1991 were used to analyze the excess risk of deaths and injuries requiring hospitalization imposed by older drivers on other road users. Analyses were conducted on aggregated data using age-group level exposure data, and on individual-level data without measures of exposure but richer information about the circumstances of individual crashes. Both approaches indicate that drivers aged

Acknowledgements

This research was conducted while the author was an Epidemic Intelligence Service Fellow at the National Center for Injury Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA. Thanks are gratefully extended to Trudy Karlson and Chip Quade of the Center for Health Systems Research and Analysis at the University of Wisconsin for providing the CODES data and general helpfulness. Jeff Sacks and Jane Stutts provided helpful comments on early drafts of this paper.

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