«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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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 COMPONENTS ON 16- AND 17-YEAR-OLDDRIVERS’ FATAL CRASHES
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 of any other age group, including older teens (Insurance Institute for Highway Safety 2012). Graduated driver licensing (GDL) has been the single most effective intervention to reduce motor vehicle related injury and fatality in this population (Insurance Institute for Highway Safety 2012). GDL is based on the premise that the mastery of any complex task requires time and extended practice. All beginning drivers are inexperienced and prone to making driving errors, and therefore at a higher risk of crashing (Waller 2003). GDL shifts the focus from providing specific, detailed training to individuals directly, and exerts its influence through modifying the driving environment for novice drivers in a way that reflects the reality of learning a complex task (Foss 2007).
As a system, GDL is based on learning theory, which asserts that mastery of a skill requires extended practice, and a gradual move from simple to complex conditions (Waller 2003). An ideal GDL system includes three licensure levels (Foss and Goodwin 2003), and as novice drivers advance through these levels, they experience the principles of GDL in action. 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). The period of supervised driving has the lowest lifetime crash risk (Mayhew, Simpson et al. 2003; VicRoads 2005), meaning that learner license holders gain their initial driving experience under very safe conditions. All driving within the first level of licensure occurs under supervision, however, some states mandate a specific number of supervised driving hours to be completed (e.g., 30 hours). Most states also require a sixmonth learner license. A few require less, and North Carolina requires 12 months (Insurance Institute for Highway Safety 2012).
The second level (intermediate license) allows teens who have gained some 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 stage of GDL 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. Increasing driving privileges by easing restrictions at each stage of licensure allows a gradual move from simple to more complex driving conditions.
There is little question that GDL reduces young drivers crashes (Insurance Institute for Highway Safety 2012), however, it is not clear how these reductions are achieved. McKnight and Peck suggest the safety effect of GDL is achieved by limiting driving to the lowest risk conditions (supervised driving) and through extending the period of practice driving (McKnight and Peck 2002). 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 little is known about both the optimal number of months a learner license should be held, and the optimal number of required supervised driving hours.
Studies in Kentucky, Connecticut, and Nova Scotia indicate substantial crash reductions for 16-year-old drivers when a learner license period was mandated or an existing period was extended. In Kentucky, crash rates of 16-year-old drivers dropped by 33% when the learner license duration was extended from 30 days to six months (Agent, Steenbergen et al. 2001). This effect was primarily attributable to an 83% crash reduction among those aged 16 to 16 years and six months, who would be driving exclusively with a learner permit. Fatal and injury crash involvements of Connecticut 16year-old drivers declined by 22% in the first year following the introduction of a six-month learner license period (or four months with driver education, which is optional in that state) (Ulmer, Ferguson et al. 2001). In Nova Scotia, the crash rate for 16- and 17-yearold GDL novices was 50% lower than the rate for pre-GDL novices when the learner license was extended from 60 days to six months (Mayhew, Simpson et al. 2003).
While these findings indicate an extension of the learner license period reduces crashes, none of these studies used licensure data, so the specific mechanism (delay in licensure or safer independent driving) by which crash reductions were achieved is unclear. Little is also known about the optimal number of months a learner license should be held for the best safety benefit. In all three states above, the learner license was extended to or mandated to be six months, however, there is no evidence to suggest whether or not a six-month period of supervised driving is adequate (Foss 2007). For example, for the states discussed above, it is unknown whether a doubling of the learner license period (to twelve months) would have resulted in the same or a larger crash reduction.
Less is known about the safety effect on teen drivers of the required number of supervised driving hours. The small body of research examining the subject is inconclusive. A study of Swedish teens found an average of 120 hours of supervised driving was associated with a significant reduction in crash involvement during independent licensure, compared to those who had approximately 50 hours of supervised driving practice (Gregersen, Berg et al. 2000; Gregersen, Nyberg et al. 2003;
Sagberg and Gregersen 2005). However, teen drivers in the northeastern U.S. who completed a period of supervised driving were no different in their time-to-first-crash than those who did not have supervised driving experience (McCartt, Shabanova et al. 2003).
Similarly, French teens who received professional driving instruction with an extensive period of supervised driving (equivalent to approximately 3,000 miles) had the same crash likelihood as those teens who only received professional driving instruction (Page 2004).
The Swedish study that found an average of 120 hours of supervised driving reduced teen drivers’ crashes is notable because it is the only instance where the effectiveness of supervised driving hours has been reported, and the number of supervised driving hours was considerably longer than any existing requirement in the U.S., which is 60 hours (in both Kentucky and Maryland (Insurance Institute for Highway Safety 2012)). However, due to the small number of studies, it is not possible to determine whether 120 hours is the optimal number of supervised driving hours, or whether a greater number of supervised driving hours would result in the same or a larger crash reduction.
States that changed the number of months a teen holds a learner license or the required number of supervised driving hours, independent of any other GDL component, represent natural experiments where intervention effects can potentially be measured. In such cases, inferences regarding the impact of the change on teen crash rates would be more strongly supported than in situations where multiple GDL components were changed simultaneously. While it has been argued that quantifying the contribution of individual components of GDL is difficult (Foss 2007), estimating their specific contribution in such natural experiments represents an opportunity to understand the mechanisms through which GDL exerts its effect.
The purpose of this paper is to quantify the effect of two required components of the learner license on teen drivers’ fatal crash rates: the length of the learner license (months) and the number of supervised driving hours. This paper begins by proposing a series of research hypotheses relating to the effect of these two components. This is followed by a description of the research methods for the study. The results of the analysis quantifying the effect of these components on 16- and 17-year-olds’ fatal crash rates is then presented, followed by a discussion of the key findings.
In a number of jurisdictions, sizeable reductions in novice drivers’ crashes have followed the introduction of a minimum duration a learner license must be held (Agent, Steenbergen et al. 2001; Ulmer, Ferguson et al. 2001; Mayhew, Simpson et al. 2003).
We hypothesize that:
1. The introduction of the learner license minimum duration as part of GDL will be followed by a reduction in 16- and 17-year-old drivers’ fatal crash rates, and there will be a dose response relationship between the duration of the learner license and the reduction in fatal crash rates.
The number of required supervised driving hours that has demonstrated a reduction in novice drivers’ crashes is more than double the supervised driving hours required by the states in this study sample (Gregersen, Nyberg et al. 2003).
Nevertheless, we hypothesize that:
2. The introduction of a minimum number of required supervised driving hours will be followed by a decline in 16- and 17-year-old drivers’ fatal crash rates, and there will be a dose response relationship between the number of supervised driving hours and the reduction in fatal crash rates.
To test the first hypothesis, states that introduced a minimum duration of the learner license independent of other GDL components, during the period 1990 to 2009, were identified. Three states: Hawaii, South Carolina and Tennessee, mandated a learner license duration of three months, while five states: Connecticut, Kentucky, Minnesota, Virginia, and Utah, established a learner license period of six months during the time being studied. For the majority of these states, the mandatory learner license period was the first component of GDL that was implemented; however, South Carolina had existing nighttime driving restrictions when the learner license period was established (Table 2.1).
To test the second hypothesis, states that introduced a required number of supervised driving hours independent of other GDL components, during the period 1990 to 2009, were identified (Table 2.2). Six states: Arizona, Kentucky, Maine, Minnesota, New Hampshire, and Rhode Island, introduced a law independently of any other GDL components that required novice drivers to complete a specified number of supervised driving hours, ranging from 20 to 60. In Arizona and Maine, this law was the first GDL component to be implemented, while in Kentucky, Minnesota, New Hampshire and Rhode Island, the supervised driving hour requirement added to an existing GDL system.