Published online November 1, 2006
PEDIATRICS Vol. 118 No. 5 November 2006, pp. 2253-2254 (doi:10.1542/peds.2006-2385)

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LETTER TO THE EDITOR

Effects of Graduated Driver Licensing on Fatalities in 16-Year-Olds: In Reply

Guohua Li, MD, DrPH
Department of Emergency Medicine
Johns Hopkins University School of Medicine
Baltimore, MD 21209

Li-Hui Chen, MS, PhD
National Center for Health Statistics
Centers for Disease Control and Prevention
Hyattsville, MD 20782

Susan P. Baker, MPH
Johns Hopkins School of Public Health
Center for Injury Research and Policy
Baltimore, MD 21205

The letter by Desapriya et al concerns 2 issues: exposure measurement and unmeasured variables. We appreciate the opportunity to clarify them. We did not use vehicle miles traveled as our exposure measure but instead based our analysis on population of 16-year-olds, because our focus was the net benefit of graduated driver licensing (GDL) in reducing motor vehicle mortality among 16-year-olds. We did acknowledge in our discussion section1(p61) that, in the absence of exposure data, it was impossible to determine the extent to which the observed GDL-associated reduction in fatal crashes was attributable to decreased exposure to driving or decreased crash-instance density.

Confounding by unmeasured variables is a concern inherent in all observational studies, including the studies cited by Desapriya et al. Traffic safety can be affected by numerous factors related to the environment, the driver, and the vehicle. There is no way other than a randomized, experimental design to control for all the possible confounders. We approached the issue of unmeasured variables by applying the most appropriate statistical methods and by constructing the most sensible and parsimonious statistical model. Specifically, to control for unmeasured variables we included design variables indicating individual states and crash data for 2 older age groups (20–24 and 25–29 years) that were unaffected by GDL regulations but might be similarly affected by other variables such as traffic density, weather conditions, and seat belt laws. The observed GDL-associated reduction in fatal crashes among 16-year-olds cannot be plausibly explained by such policy interventions as the minimum drinking-age and zero tolerance laws, because these regulations were implemented at a different time frame and would most likely influence alcohol-related crashes that account for only a small percentage (16%) of all fatal crashes involving 16-year-old drivers.

As important as it is, evaluation research is an imperfect science. Bias resulting from unmeasured variables is only one of the issues facing researchers. More challenging is the intricacy of social and environmental factors, which is often too complex to disentangle. We agree with Desapriya et al that our study adds valuable information to the mounting evidence base for refining and enhancing GDL programs to further reduce injury mortality and morbidity among adolescents.

REFERENCE

1. Chen LH, Baker SP, Li G. Graduated driver licensing programs and fatal crashes of 16-year-old drivers: a national evaluation. Pediatrics. 2006;118 :56 –62[Abstract/Free Full Text]

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PEDIATRICS (ISSN 1098-4275). ©2006 by the American Academy of Pediatrics

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This Article Extract Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services E-mail this article to a friend Similar articles in this journal Alert me to new issues of the journal Add to My File Cabinet Download to citation manager Request Permissions Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Li, G. Articles by Baker, S. P. Search for Related Content PubMed Articles by Li, G. Articles by Baker, S. P. Related Collections Office Practice Social Bookmarking                         What's this?