Graduated Driver Licensing Programs and Fatal Crashes of 16-Year-Old Drivers: A National Evaluation

Data

Data on fatal crashes and population were obtained from 2 federal sources: the Fatality Analysis Reporting System (FARS) of the National Highway Traffic Safety Administration¹³ and the US Census Bureau.¹⁴ FARS is a census of all fatal traffic crashes within the United States that involve a motor vehicle traveling on a public road and result in a death within 30 days of the crash. The numbers of drivers aged 16 and aged 20 to 29 years involved in fatal crashes in each state for each month from 1994 through 2004 were obtained from FARS. Cases where two 16-year-old drivers were involved in the same crash were counted as 2 events. Midyear population estimates for each state from 1994 to 2004 were obtained from the US Census Bureau. Information on GDL programs and their effective dates was obtained from the Insurance Institute for Highway Safety,¹⁵ state government Web sites, and personal contacts with state personnel.

Although each GDL program has some distinct features, the main provisions in the various GDL programs generally fall into 7 categories: minimum age for a learner permit, mandatory waiting period before applying for intermediate license, minimum hours of supervised driving, minimum age for intermediate license, nighttime restriction, passenger restriction, and minimum age for full licensing (Table 1). To avoid small numbers, related provisions were collapsed into dichotomous variables, for example, “nighttime driving restrictions, yes/no.”

The 7 components of GDL programs were coded into quarters of the year based on their effective month. For example, in Alabama, unsupervised driving was prohibited from midnight to 6 am beginning October 1, 2002; the first 3 quarters of 2002 were coded as not exposed to the nighttime driving restriction and the fourth quarter coded as exposed. If a restriction became effective at any time during a quarter, that entire quarter was coded as exposed. Quarters were used rather than calendar years, because GDL programs became effective at different times of the year. The unit of analysis, therefore, became the state quarter, which represents 1 state having a specified GDL component for a given quarter.

Excluded from analysis were 4 quarters after the effective date of each GDL program or component, because licensing restrictions would not affect teenagers who already had their licenses when legislation took effect. After a restriction goes into effect, it can be as long as a full year before all 16-year-old drivers in a state are driving under that restriction. Four quarters before the effective date were also excluded, because some teenagers might hasten to get their licenses before the law changed, leading to an increased number of crashes in those quarters.

The analyses included 43 states in the continental United States; 36 had GDL programs for at least part of the studied period. Washington DC was excluded from analysis, because its crash data were heavily influenced by neighboring states. Maine, New Hampshire, Rhode Island, Utah, and Virginia were excluded, because they changed their laws more than twice between 1994 and 2004, thus complicating any analysis.

The number of person-years for each quarter in each state was estimated using the midyear population of 16-year-olds divided by 4. The same method was used to calculate exposure data, that is, person-years in each state quarter for 20- to 24-year-old and 25- to 29-year-old drivers. Within the 11-year period 1994 through 2004, 1480 state quarters were examined.

Analysis

The association between GDL programs and fatal crash incidence was assessed using the negative binomial regression models based on generalized estimating equations.^16,17 The negative binomial distribution approximates the counts of fatal crashes within state quarters, and the generalized estimating equation approach takes into account the correlation among quarterly counts of fatal crashes in a given state. Statistical software SAS (SAS Institute, Cary, NC) was used for the analysis.¹⁸

Dependent Variable

The outcome variable was the natural logarithm (the unit used for all of the negative binomial models) of the number of fatal crashes involving any drivers in our target age groups in a given state quarter. States and quarters as dummy variables and year as a continuous variable were included in each model. The state variables controlled for state-specific unmeasured variations that might affect fatal crash counts, such as weather, traffic environment, regulations other than GDL, and socioeconomic conditions. The quarter variables controlled seasonal variations, and the year variable controlled for variation in fatal crash counts over the time period studied. The sum of person-years in each state quarter was considered as exposure and was included in each model.

Fatal crash involvement of drivers was used rather than driver fatalities, because it allowed more cases to be included for study. Person-years based on the population of 16-year-olds were used, because the licensing rate might have changed because of implementation of GDL programs. Also, the overall benefit to the population, regardless of the licensing rate, was of primary interest.

Independent Variables

The independent variable of primary interest was the presence or absence of GDL and its provisions. Three different approaches were used to characterize the GDL programs. In the first approach, whether a state included an intermediate phase in its licensing system was used to determine the presence or absence of GDL programs as a dichotomous variable. The reference group for this comparison was state quarters without 3-stage GDL programs.

The second and third approaches focused on the 7 GDL components studied. The programs were categorized solely based on their components, without considering whether the programs included an intermediate phase. This made it possible to examine the impact of programs that only partially met the GDL definition, thereby helping to identify the components of a good GDL program.

In the second approach, the licensing system for young drivers in each state quarter was characterized on the basis of how many of the 7 GDL components studied were contained in the licensing restrictions. The reference group was state quarters that did not meet the requirements of any of the 7 components that we examined.

In the third approach, the licensing systems for young drivers were grouped based on combinations of the 4 GDL program components not related to age of licensing: minimum waiting period of 3 months before applying for an intermediate license, minimum supervised driving of 30 hours, any nighttime restrictions, and any passenger restrictions; programs with only age restrictions were treated as an independent category. If a program grouping existed in <50 state quarters, then it was not treated as a separate category but was combined with other groupings that occurred too infrequently for separate analysis. Again, the reference group was state quarters that did not meet the requirements of any of the 7 components.

Three models based on the 3 approaches described above were fitted for each of the 3 age groups studied: drivers aged 16, 20 to 24, and 25 to 29 years. The focus of our study was 16-year-old drivers. Fatal crash-involved drivers aged 20 to 24 and 25 to 29 years were analyzed for comparison. Theoretically, GDL programs will not affect the older age groups, and, therefore, their incidence rate ratios (IRRs) should be equal to 1.

National Reductions

Results of this research provide a national overview of the association between various GDL programs and fatal crashes of 16-year-old drivers, the drivers most affected by GDL implementation. The most comprehensive programs were associated with reductions of ∼20% in crash rates of 16-year-old drivers, adjusted for differences over time and among states. For all of the full GDL programs combined, implementation was associated with an 11% reduction in fatal crashes involving 16-year-old drivers when compared with state quarters without GDL. This overall reduction reflects the combined results for all of the 3-stage GDL programs. Including states with relatively weak programs dilutes the reduction; even so, 11% is a substantial and important overall reduction.

Other investigators interested in crashes of teenage drivers have taken different approaches to assessing progress. Williams et al,¹⁹ analyzing the trend in per capita fatal crash rates of 16-year-old drivers in the United States between 1993 and 2003, without regard for GDL implementation, reported a 26% drop during this decade. The finding by Dee et al¹¹ of a reduction of only 6% associated with GDL programs may underestimate the effectiveness of GDL in reducing fatalities, because 16- and 17-year-old drivers were affected very differently by GDL programs; combining results for 16- and 17-year-old drivers may have shown a smaller reduction than would have been found specifically for 16-year-olds. Also, they did not exclude results for the 4 quarters after GDL restrictions took effect. Shope and Molnar⁵ pointed out that in the first year after implementation, the law applied to only about two thirds of the 16-year-old drivers.

Our analyses showed that programs having <5 of the 7 major components (including programs that did not qualify for 3-stage GDL programs) were not associated with significant reductions in fatal crash rates of 16-year-old drivers when compared with state quarters with none of the 7 components, whereas a reduction of 18% to 21% was associated with programs having ≥5 components. This result is similar to the 19% reduction reported for programs meeting the Insurance Institute for Highway Safety criteria for good programs.^11,12

Comparing Specific Programs

Unlike most other countries, in the United States, GDL programs vary among states. This offers a special opportunity for comparing the impact of GDL programs with different combinations of restrictions. This is the first time that analysis of the association between fatal crash involvement rates and GDL programs with specific groups of components has been reported. Our analysis of programs with specified groupings of components (without considering age criteria) revealed that combinations that included a mandatory waiting period of ≥3 months before the intermediate phase, a nighttime driving restriction, and either ≥30 hours of supervised driving or a passenger restriction were associated with reductions of 16% to 21% in fatal crashes of 16-year-old drivers. Drivers aged 20 to 24 or 25 to 29 years did not experience significant reductions in fatal crash involvement rates, suggesting that the reductions in fatal crashes of 16-year-old drivers were independent of non-GDL changes in policies or the driving environment that affected all drivers. GDL programs with only age criteria were not associated with reductions in crash rates.

According to Williams and Ferguson,²⁰ the effectiveness of GDL programs in reducing crash risk depends on addressing both age and inexperience. They suggested 3 mechanisms underlying the safety benefit of GDL programs: raising the licensing age, increasing the length of the low-risk supervised learner period,²¹ and reducing high-risk driving after initial licensure. Research on individual states suggests that the minimum age components are associated with crash reductions because they delay age of full licensure and, therefore, reduce 16-year-old drivers’ exposure to driving. For example, Shope et al³ found a substantial reduction in the number of 16-year-olds obtaining licenses after Michigan adopted a GDL program. It is clear that part of the safety benefit of GDL is because of reduced exposure to driving,²² which by itself can be expected to lead to reduced crashes and injuries.²³ Our analysis of the number of components of driver licensing systems also suggested that age of licensure is important, because without age components, a program would not have ≥5 components, the number needed to make a significant difference. In addition, of course, the age components delay licensure, which is important, because it can reduce exposure of 16-year-olds.

Our analysis indicates that GDL components intended to reduce high-risk driving at night or with teenage passengers after initial licensure contribute to the effectiveness of GDL programs. It is not possible to discern whether the association that we observed is directly because of enforcement of the nighttime driving and passenger restrictions or whether parent-imposed limits on high-risk driving of 16-year-olds are stricter in states with more restrictive programs. The importance of parental involvement cannot be overestimated, and Simons-Morton et al²³ state that future reductions in teen driver crashes may depend on increasing parental management. Hartos et al²⁴ reported that parents seem better able to establish and enforce teenage driving restrictions when state laws support them. Whatever the mechanism by which nighttime and passenger restrictions are associated with crash reductions, a program with those components seems to be desirable.

Limitations

In the absence of data on age-specific driver populations and time spent in driving, we were unable to determine the extent to which GDL-associated reductions in fatal crashes involving 16-year-old drivers were because of reduced exposure associated with decreased licensure or because of decreased driving time and distance.

Also, it was not possible to distinguish the benefits of a law, per se, from benefits of implementation, enforcement, and compliance with that law. When a particular GDL program is not associated with a reduction in crash involvement, it is likely that compliance is low, and this could be because of flaws in the policy or the environment of the policy, such as publicity, enforcement, and parental involvement. Some restrictions are easier to enforce than others. Requirement of a 3- or 6-month waiting period is virtually always enforced, because it is an integral part of how the licensing system functions, rather than depending on the actions of tens of thousands of individual parents, whereas certified supervised driving will largely depend on the willingness and ability of parents to supervise. A night driving restriction is far easier for parents to enforce than a passenger restriction. Goodwin and Foss²⁵ surveyed teenagers and their parents in North Carolina and confirmed that violation of restrictions without parental knowledge was more common for passenger restrictions than nighttime restrictions.

Another limitation was that some groups of GDL components were present in too few state quarters for analysis, which could have prevented identification of successful programs. Also, small numbers made it impossible to use more detailed categories for GDL components. For example, we were not able to determine whether results varied with the number of passengers allowed or the permitted age for supervisors. Previous studies have indicated that crash risk of teenage drivers increased with the number of passengers.^26,27 The importance of supervisor age is underscored by data from Chen et al,²⁷ who found the highest case-fatality rates of 16-year-old drivers in crashes when passengers aged 20 to 29 years were present. This might be related to the fact that older passengers may legally buy alcohol and (illegally) provide it to underage drinkers, although Rice et al²⁸ indicated that the presence of adults aged 20 to 29 was associated with severe or fatal injury among 16- and 17-year-old drivers even when alcohol use was controlled for. Williams and Shabanova²⁹ reported that teen drivers were less likely to use seat belts when passengers were in their 20s, and recommended that passenger restrictions not be waived unless there is a supervisor ≥30 years of age. This recommendation is reasonable, because mature passengers are more likely to take some responsibility for the safety of a trip.

Finally, our estimates may underestimate the benefit of GDL, because we assumed that a restriction imposed at any time during a 3-month period became effective at the beginning of the period. Any effect of this assumption is likely to be small, because most restrictions do, in fact, become effective at the beginning of a calendar quarter and because the analysis excluded data for the entire year after the effective date of each GDL requirement.