| 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 single-state 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 passenger crash rates that approached significance (p= .06). Nighttime driving 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. |
