«by Johnathon P. Ehsani A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Health Behavior ...»
It was hypothesized that the implementation of a required number of supervised driving hours would be associated with a decline in teen drivers’ fatal crashes. The increase in teen drivers’ fatal crash rates in Minnesota was unexpected and should be interpreted with caution. Given that an increase in teen drivers’ fatal crashes following the introduction of required supervised driving hours was not observed in any other state in the sample, this result may be due to state level differences in compliance and enforcement, or the unique sequence and combination of laws that was implemented in Minnesota. The increase in crashes may also be the result of an increase in the number of learner licenses issued, as teens may have rushed to begin driving before this GDL component came into effect, a phenomenon that has been observed in other states (Masten and Hagge 2004). However, licensure data for Minnesota teens was not available to confirm this hypothesis. A recent study testing the effect of the supervised driving hour requirement in Minnesota using all injury crashes (not just fatal crashes) found no association between the law and crash rates (The University of North Carolina Highway Safety Research Center In Press).
This study adds to the literature by finding that a learner license period that guarantees a six-month delay in licensure is associated with a significant decline in teen drivers’ fatal crash rates. Although the primary objective of the learner license is to allow for an extended period of practice driving under safe conditions, the findings of this study suggest the decline in crashes associated with the learner license are primarily due to delayed licensing. A learner license period that guarantees license delay by 6 months is likely result in a decline in teen crashes in those states where the learner license periods do not currently guarantee a delay. However, this study could not establish whether a doubling of the learner license period (to twelve months) would result in the same or a larger crash reduction. Nor could we establish an association between the number of required supervised driving hours and fatal crashes.
These findings were based on fatal crashes involving drivers. Fatal crashes represent a small and atypical subset of all crashes, where the etiology of fatal crashes differs from that of less serious crashes (Lam 2003). Significant changes in fatal crash rates were observed in large states, suggesting the absence of an effect in smaller states was due to insufficient power. Four states in the sample have relatively small populations (Hawaii, Maine, New Hampshire and Rhode Island) and fatal crashes are rare events, increasing the probability of a floor effect, (where crash rates cannot take on a value lower than zero). However, estimating the models for these states using quarterly data, which reduced the number of time points but potentially increased variation in the series, did not alter the results that were based on monthly data.
An alternative explanation is that in the majority of cases, the implementation of a single GDL component did not result in a significant reduction in fatal crashes. Research by Chen and Baker suggests that teen drivers’ fatal crashes decline significantly when three or more GDL components are in effect (Chen, Baker et al. 2006). Similarly, Morrisey and colleagues found that states with GDL legislation that was rated by the Insurance Institute for Highway Safety as ‘good’ (i.e., more, stronger components) had significantly lower teen driver fatal crashes compared to states with ‘fair’ or ‘poor’ rated GDL (i.e., fewer, weaker components) (Morrisey, Grabowski et al. 2006; McCartt and Teoh 2011). Given that eight of the thirteen states in this study implemented required driving hours or a learner license period as the first element of GDL, independently of other components, it seems plausible that the introduction of these laws was not enough to be associated with a reduction in teen drivers’ fatal crashes.
Future research examining the effect of GDL learner license requirements (duration of the learner license period or number of supervised driving hours) on teen drivers should extend the analysis to include all crash types: property-damage-only crashes, injury crashes, as well as fatal crashes. While it may be the case that an extension of the learner license holding period has the same effect on non-fatal and fatal crashes, this approach has not been tested empirically. The same is true for requiring a minimum number of supervised driving hours. Currently, only a limited number of states make non-fatal injury crash data available to researchers (National Highway Traffic Safety Administration 2011), yet the largest proportion of teen drivers’ crashes is non-fatal.
These data would allow an examination of the effects of GDL on crash types different severity.
Table 2.1: States that introduced a learner license period independent of other GDL components
Minnesota 6 month learner license (2/97) 40 hours supervised driving (1/99) Kentucky 6 month learner license (10/96) 60 hours supervised driving (10/06)
* Age-group-specific annual fatal crash involvement rates of drivers per 100,000 population, were estimated by averaging the monthly fatal crash rate, calculated using monthly fatal crash counts and monthly population estimates for each age group.
Component of interest highlighted in bold Table 2.5. Parameters of best-fitting sudden permanent change ARIMA models estimating the effect of supervised driving hours on 16- and 17- year-olds’ fatal crash rates per capita, 1990 – 2009
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Motor vehicle crashes are the leading cause of death and a leading cause of injury for teenagers in the United States (National Highway Traffic Safety Administration 2010). Sixteen-year-old drivers in their first year of licensure have higher crash rates than any other age group, including older teens (Insurance Institute for Highway Safety 2012). Teen drivers’ fatal crash risk is particularly high while driving with peer passengers and/or at night (Williams, Preusser et al. 1995; Chen, Baker et al. 2000).
Teen drivers’ fatal crash risk increases with each additional peer passenger in the vehicle. The relative risk of death among 16- to 17-year-olds who have one peer passenger in the vehicle is significantly greater than when driving alone. Carrying at least three peer passengers results in a threefold increase in the likelihood of a fatal crash (Chen, Baker et al. 2000). Driving between the hours of 9 p.m. and 5 a.m.
accounts for approximately 15% of 16- to 17-year-olds’ total miles driven, however, 40% of teen drivers’ fatal crashes occur between these hours (Williams and Pruesser 1997).
Graduated driver licensing (GDL) eases young drivers onto roadways by limiting their exposure to progressively higher risk driving experiences (National Highway Traffic Safety Administration. U.S. Department of Transportation 1996). The move from simple to increasingly complex driving conditions is mediated by teens’ progression through three licensure levels (Foss and Goodwin 2003). The first level (learner license) allows teens to gain driving practice and experience under the supervision of a fully licensed driver (typically a parent or other person over age 21 who is designated by the parent).