«by Johnathon P. Ehsani A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Health Behavior ...»
National studies of GDL have also reported significant declines in 16- and 17year-old drivers’ crashes corresponding to the presence of a nighttime driving restriction.
McCartt found the presence of a nighttime driving restriction was significantly associated with a reduction in 16- and 17-year-old drivers’ fatal crashes (McCartt, Teoh et al. 2010).
Similarly, Trempel observed that the effect of a nighttime driving restriction was associated with significantly fewer insurance collision claims by 16- and 17-year-old drivers (Trempel 2009). Karaca-Mandic found that GDL programs that included a nighttime driving restriction reduced fatal crashes by 15% (Karaca-Mandic and Ridgeway 2010). Recently, Fell reported a 10.3% decline in 16- and 17-year-old drivers’ nighttime fatal crashes following the introduction of nighttime driving restrictions (Fell, Todd et al. 2011).
Collectively, these studies present a convincing body of evidence demonstrating the effectiveness of the nighttime driving restriction. However, in the majority of instances, evaluations of nighttime driving restrictions have employed a pre-post-GDL study design. As a result, the authors have not been able to distinguish a decline directly attributable to GDL from a continuation of a preexisting downward trend (Sivak and Schoettle 2010). For example, a follow-up study of the effect of Michigan’s nighttime driving restriction found that changes in nighttime crash rates among 16-year-old drivers in Michigan could not be attributed to GDL, but that the downward trend in nighttime crashes began a year prior to the introduction of GDL and continued as a linear trend throughout the GDL introduction period (Elliott and Shope 2003).
Similar to passenger restrictions, in the majority of instances when nighttime driving restrictions were introduced in the United States, they were implemented along with at least one additional GDL component. State and national evaluations have rarely accounted for the confounding effect of multiple GDL components implemented simultaneously. Therefore, it is possible that existing estimates of the effect of nighttime driving restrictions may be overestimated due to issues related to study design and the misapplied assumption of independent implementation.
One approach to quantifying a more precise measure of the effectiveness of nighttime driving restrictions on 16- and 17-year-old drivers’ crashes is to identify instances when a nighttime driving restriction was implemented independently of other components. In such cases, inferences regarding the impact of GDL on teen crash rates would be more strongly supported than in situations where multiple GDL components were changed simultaneously.
Effect of GDL on 18-year-old drivers With few exceptions, the staged licensing requirements of GDL apply exclusively to 15- to 17-year-old drivers (Williams and Shults 2010). In most U.S. states, individuals 18 year of age and older wishing to obtain a driver’s license for the first time are required to complete a knowledge test and a driving skills test prior to receiving a regular license, effectively bypassing the learner, intermediate, and full license stages of GDL.
While a sizeable body of evidence suggests that GDL leads to significant declines in 16- and 17-year-old drivers’ crashes (Foss, Feaganes et al. 2001; Shope, Molnar et al. 2001; Williams, Ferguson et al. 2005; Chen, Baker et al. 2006; Trempel 2009; McCartt, Teoh et al. 2010; Williams and Shults 2010; Masten, Foss et al. 2011), the effect of GDL on 18-year-old drivers’ crashes is less clear. A study examining the effect of California's GDL on teen driver fatalities reported a 24% rise in 18-year-old drivers’ fatal crashes following the introduction of GDL in July 1998 (Males 2007). A recent national study by Masten and Foss also reported a significant increase in fatal crashes of 18-year-old drivers following the introduction of GDL that was large enough that there was no net benefit from GDL on overall 16- to 19-year-old drivers’ fatalities (Masten, Foss et al. 2011). In contrast, McCartt and colleagues found that states with GDL programs that were rated by the Insurance Institute for Highway Safety as ‘good’ (i.e., more, stronger components) had significantly lower 18-year-old driver fatal crashes compared to states with ‘fair’ or ‘poor’ rated GDL programs (i.e., fewer, weaker components) (McCartt, Teoh et al. 2010; McCartt and Teoh 2011). Using the same taxonomy to rate GDL programs, Trempel found that ‘good’ GDL programs were associated with significantly fewer insurance collision claims by 18-year-old drivers (Trempel 2009).
These studies differed in the quality of methodological approaches that were employed and the data sources that were used. The studies reporting an increase in 18year-old drivers’ fatalities used time series analysis, an evaluation method that controls for pre-existing trends, seasonal variation and serial correlation between observations (McCleary and Hay 1982). Masten’s national evaluation pooled individual states’ time series into a single sample, making this evaluation arguably the most complete study on the effect of GDL that has been conducted to date. However, both studies utilized data from the Fatal Analysis Reporting System (FARS) and, therefore, were limited to only fatal crashes (National Highway Traffic Safety Administration 2010). Neither McCartt (McCartt, Teoh et al. 2010) nor Trempel (Trempel 2009) controlled for pre-existing trends in teen crashes in their analyses, and McCartt was limited to fatal-only crashes.
Futhermore, Trempel’s analysis was restricted to collision claims involving new (i.e., 0-3year-old) motor vehicles (Trempel 2009), a sample that is known to be unrepresentative of teen drivers (Williams, Leaf et al. 2006).
Dissertation Significance The purpose of this dissertation was twofold. The first was to quantify the effect of each component of GDL on 16- and 17-year-old drivers’ crashes. The second was to quantify the overall effect of GDL on 18-year-old drivers’ crashes, and shed light on the mechanisms responsible for some potential increase in crash rates in this population. By exploiting natural experiments in GDL policy implementation and using time series analysis, we estimated the effects of GDL on teen drivers’ crashes with less confounding than in previous studies.
This dissertation includes five chapters in total. An introductory chapter (Chapter
1) and a concluding chapter (Chapter 5) and three research papers (Chapters 2 – 4).
Paper 1 (Chapter 2) examines the effect of the learner license requirements (i.e., learner license period and supervised driving requirement) on 16- and 17-year-old drivers’ fatal crash involvement. Paper 2 (Chapter 3) examines the effect of intermediate license restrictions (i.e., nighttime driving restriction and passenger restriction) on 16and 17-year-old drivers’ fatal crash involvement. Paper 3 (Chapter 4) examines the overall effect of GDL on 18-year-old drivers’ injury crash rates. Chapter 5 concludes with an integrated discussion of the research findings, including strengths and limitations of the studies, and implications for future research.
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