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«by Johnathon P. Ehsani A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Health Behavior ...»

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Johnathon P. Ehsani

A dissertation submitted in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

(Health Behavior and Health Education)

in The University of Michigan

Doctoral Committee:

Research Professor C. Raymond Bingham, Co-Chair

Professor Marc A. Zimmerman, Co-Chair Professor Peter X. Song Associate Professor Cleopatra H. Caldwell Research Professor Jean T. Shope © Johnathon P. Ehsani DEDICATION To the safety and well-being of all young people ii


This dissertation research would not have been possible without the support and encouragement of many people. In particular, my parents, who gave me their blessing to pursue doctoral studies so far away from home; my wife, who has endured endless hours of speculation on how graduated driver licensing works and why; my father-in-law, who has accompanied me every step of the way; the members of my dissertation committee for joining me on this journey, and providing valuable guidance; in particular, Ray Bingham and Jean Shope who have mentored me in complementary and equally profound ways; and finally, the Center for Injury Prevention among Youth at the University of Michigan, with support from University of Michigan Transportation Research Institute, and the Office of the Vice President for Research; and the General Sir John Monash Foundation for giving me the opportunity to pursue my calling.



Dedication ……………………………………………………………...………………………ii Acknowledgements…………………………………………………...…………………………iii List of Tables……………………………………………………………...……………………...v List of Abbreviations…………………...…………………………...………………..…………vii Abstract……………………………………………………………...……………………….….viii Chapter 1: Introduction……………………………………………………………..…………..1 Chapter 2: Does The Length Of The Learner License Period Or The Hours Of Supervised Driving Matter? An Analysis Of The Effect Of Two Graduated Driver Licensing (GDL) Components On 16- And 17-Year-Old Drivers’ Fatal Crashes………………………………………………………………………...…………..……22 Chapter 3: Do Passenger And Nighttime GDL Restrictions Work? An Analysis Of Their Independent Effects On 16- And 17-Year-Old Drivers’ Fatal Crashes………..…...50 Chapter 4: Delaying The Inevitable? The Effect Of GDL On 18-Year-Old Drivers’ Crashes In Florida, Michigan And Maryland………………………………..…………..…...80 Chapter 5: Conclusion……………………………………………….………..………….….107

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Table 2.1 States that introduced a learner license period independent 38 of other GDL components.

Table 2.2 States that introduced a required number of supervised 39 driving hours independent of other GDL components.

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Table 3.2 States that added an intermediate license nighttime driving 67 restriction independently of other GDL components between 1990 and 2009.

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Purpose: The purpose of this dissertation was to answer the following questions:

1. What is the effect of each component of Graduated Driver Licensing (learner license duration, required hours of supervised driving, passenger restrictions and nighttime driving restrictions) on 16- and 17-year-old drivers’ fatal crash rates?

2. What is the effect of GDL on 18-year-old drivers’ crash rates, and what mechanisms might be responsible for any increase in rates?

Method: To answer question 1, states that introduced a single GDL component, independent of other components were identified. The effect of the single GDL component on 16- and 17-year-old drivers fatal crashes was estimated using singlestate time series analysis, adjusting for adult crashes and gas prices.

To answer question 2, single-state time series analysis was used to estimate the effect of GDL on 16-, 17-, and 18-year-old drivers’ crashes in Florida and Michigan, where GDL applies to 15- to 17-year-old drivers, and in Maryland, where GDL applies to novice drivers of all ages, adjusting for adult crashes and gas prices.

Results: A learner license period that guaranteed six-months delay in licensure to drive independently was associated with a significant decline in 16- and 17-year-old drivers’ fatal crash rates. In one state, novice drivers’ fatal crash rates increased 34.5% following the introduction of 30 hours of required supervised driving. A passenger restriction for the first 12 months of intermediate licensure was followed by a 46% reduction in fatal

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restrictions, implemented alongside supervised driving hours, did not reduce fatal nighttime crashes. The introduction of GDL was followed by a significant increase in possible-injury/property-damage-only crashes among 18-year-old drivers in Michigan and by a significant decrease in possible-injury/property-damage-only crashes among 18-year-old drivers’ rates in Maryland.

Conclusion: Some GDL components confer a safety benefit. However, the entire program is responsible for a greater reduction in crashes than the additive contribution of individual components. GDL programs applied exclusively to 16- and 17-year-old drivers may result in some teens not being licensed until age 18. Requiring all novice drivers to complete a GDL program is recommended.

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Motor vehicle crashes (MVCs) are the leading cause of death and a leading cause of injury for teens in the United States (National Highway Traffic Safety Administration 2010). Nationally, teens are overrepresented in MVC deaths. Crash rates per mile driven for 16- to 19-year-olds are four times the rates for adult drivers (Insurance Institute for Highway Safety 2012). For every MVC death among 16- to 19year-olds, there are an estimated 10 serious injuries requiring hospitalization and 178 minor injuries (Christoffel and Gallagher 2006). The Centers for Disease Control and Prevention has identified the prevention of teen MVCs a winnable battle, and Graduated Driver Licensing (GDL) programs have been advanced as the centerpiece of this strategy (Frieden 2011).

Graduated Driver Licensing The original basis for proposing GDL was crash data from the early 1970s demonstrating teen divers were overrepresented in crashes in relation to their presence on the road, particularly during the hours of midnight and 6 a.m., and when peer passengers were present in the vehicle (Waller 2003). MVCs had emerged as the leading cause of fatal crashes among younger drivers two decades earlier, when deaths from infectious diseases such as tuberculosis had been effectively eliminated (Centers for Disease Control and Prevention 2009). However, it was not until the 1990s that GDL became widely adopted throughout the United States (Insurance Institute for Highway Safety 2012).

GDL is based on the premise that the mastery of driving requires time, extended practice, and a gradual progression from simple to complex driving conditions (Waller 2003). As teens gain driving experience, they graduate to progressively higher-risk driving conditions. The move from simple to increasingly complex conditions is mediated by teens’ progression through three licensure levels.

The first level (learner license) allows teens with the least driving experience to gain practice under the supervision of a fully licensed driver (typically a parent or some other person over the age of 21 and designated by the parent). Most states require teens to hold a learner license for six months or longer. Driving during the first level of licensure occurs under supervision, meaning that learner license holders gain their initial driving experience under very safe conditions (Mayhew, Simpson et al. 2003). However, some states mandate a specific number of supervised driving hours to be completed (e.g., 30 hours) as part of the requirements of the learner license period.

The second level (intermediate license) allows teens who have gained initial experience driving with a learner license, to drive independently but with some restrictions that limit their exposure to the highest risk driving conditions (driving at night (Williams 2003) and driving with peer passengers (Chen, Baker et al. 2000)). The third level of GDL (full license) gives teens who have gained driving experience over a protracted period while fulfilling the requirements of a learner and intermediate license, permission to drive with no restrictions.

As novice drivers advance through these levels, they experience the principles of GDL in action. Each level is distinct in the requirement or restriction it demands from teen drivers. During the first level of licensure, teens are required to hold a learner license and complete a minimum number of supervised driving hours. During the second level of licensure, teens are restricted from driving with passengers, or driving at night.

Each requirement or restriction is referred to as a component of GDL. Comprehensive GDL systems include three licensing levels and three or more components (Foss 2007).

Effectiveness of Graduated Driver Licensing Evaluations of GDL have demonstrated the effectiveness of this licensure system in reducing novice teen drivers’ crash involvement. Without question, GDL has been the single most effective intervention in reducing motor vehicle-related injuries and fatalities among 16- and 17-year-olds (Insurance Institute for Highway Safety 2011). However, how GDL produces its risk-reducing effect, or more precisely, the contribution of each component of GDL (learner license duration, supervised driving hours, passenger restriction, and nighttime driving restriction) to crash reduction remains unclear (Shults, Begg et al. 2010).

Also unclear is the effect of GDL on 18-year-old drivers. Some studies indicate that GDL reduces 18-year-old drivers’ crashes (Trempel 2009; McCartt, Teoh et al.

2010); while two recent studies suggest that GDL is associated with increased fatal crashes of 18-year-old drivers, such that there is no net benefit of GDL on crash fatalities for teen drivers overall (Males 2007; Masten, Foss et al. 2011). These findings raise questions about possible unintended consequences of GDL, and potentially threaten existing GDL systems that have demonstrated significant public health benefit, so a greater understanding is needed.

Establishing the effectiveness of each component of GDL on 16- and 17-year-old drivers’ crashes, and the effect of GDL on 18-year-old drivers’ crashes have been identified as research priorities (Hedlund, Shults et al. 2003; Shope 2007; Williams and Shults 2010). The purpose of this dissertation was to address these questions, namely to quantify the effect of each component of GDL on 16- and 17-year-old drivers’ crashes, and the overall effect of GDL on 18-year-old drivers’ crashes. The remainder of the introduction will review the existing literature examining these two questions.

GDL Components Since GDL was first introduced in the United States, a sizeable body of research has consistently demonstrated its effectiveness in reducing all crash types for teen drivers (Shope and Molnar 2003; Hartling, Wiebe et al. 2004; Hedlund, Shults et al.

2006). However, this introduction will demonstrate that only a few state-level and national studies have evaluated the impact of individual components of GDL, and these are limited by weaknesses in study design as well as conceptual shortcomings. Most studies have ignored the temporal correlation among components and the resulting confounding effect of multiple GDL components implemented simultaneously. In such cases, estimates of effects associated with individual components that were implemented alongside others would be too correlated with one another to allow meaningful analysis of their separate effects. As a result, the manner in which GDL exerts its safety effects is not clearly understood. Furthermore, evaluations of GDL implementation have employed a pre-post-GDL comparison. That study design cannot distinguish changes in crashes directly attributable to GDL from differences arising from a preexisting trend, resulting in lingering questions regarding contributions to observed safety outcomes. The following sections will examine the research evidence related to each GDL component, and the effect of GDL on 18-year-old drivers, and identify the gaps in the literature that remain to be addressed.

Learner License Duration Currently, the learner license requirements are the most widely implemented of all GDL components in the U.S., existing individually or side-by-side with other components in all fifty states and the District of Columbia. Therefore, it is surprising that the contribution of this component within a comprehensive GDL program is not well understood.

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