IV. DISCUSSION

A. National Differences in Fatal Crash Involvement Rates

Results of this research provide a national overview of the association between various GDL programs and fatal crash involvement rates of 16-year-old drivers – the drivers most affected by GDL implementation. Adjusting for differences over time and among States, the most comprehensive GDL programs were associated with fatal crash involvement rates for 16-year-old drivers, that were about 20 percent lower than programs without any of the seven GDL components. For all full GDL programs combined, implementation was associated with an 11-percent lower fatal crash involvement rate for 16-year-old drivers compared to State-quarters without GDL. This overall difference reflects the combined results for all three-stage GDL programs. Including States with relatively weak programs dilutes the effect; even so, 11 percent is a substantial and important overall difference.

Other investigators interested in crashes of teenage drivers have taken different approaches to assessing progress. Williams and colleagues,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-percent drop during this decade. The finding by Dee and colleagues 11 of a reduction of only 6 percent 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. This may be a smaller reduction than would have been found specifically for 16-year-olds, due to combining results for 16- and 17-year-old drivers. Moreover, the authors did not exclude results for the four quarters after GDL restrictions took effect, and given that GDL programs do not impact 16-year-olds already licensed, their finding may underestimate the effect. Shope and Molnar 5 pointed out that in the first year following implementation the law applied to only about two-thirds of the 16-year-old drivers.

Our analyses showed that programs having fewer than five of the seven major components (including programs that did not qualify for three-stage GDL programs) were not associated with significant differences in fatal crash rates of 16-year-old drivers when compared to State-quarters with none of the seven components, while a difference of 18-21 percent was associated with programs having at least five components. This result is similar to the 19-percent lower rate reported for programs meeting the IIHS criteria for ‘good’ programs.11 12

B. 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 programs that included a mandatory waiting period of at least three months before the intermediate phase, a requirement of 30 or more hours of supervised driving, and passenger and nighttime driving restrictions were associated with fatal crash involvement rates for 16-year-old drivers that were 16 to 21 percent lower when, compared to programs with none of the seven components in Table 1. Drivers ages 20 to 24 and 25 to 29 did not experience significant changes in fatal crash involvement rates, suggesting that the lower fatal crash involvement rates for 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 fatal crash involvement rates.

According to Williams and Ferguson,20 the effectiveness of GDL programs in reducing crash risk depends upon addressing both age and inexperience. They suggested three 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 al.3 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 due to 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. Indeed, without age components a program would not have five or more 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 we observed is directly due to 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 states that future reductions in teen driver crashes may depend upon increasing parental management.23 Hartos and colleagues 24 reported that parents appear 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 lower crash rates, a program with those components is clearly advantageous.

C. 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 changes in fatal crashes involving 16-year-old drivers were due to reduced exposure associated with decreased licensure or to decreased driving time and distance.

Also, it was not possible to determine whether a law was effective due to enforcement or to public support and other factors affecting compliance with the law. When a particular GDL program is not associated with a lower crash rate, it is likely that compliance is low and this could be due to flaws in the policy or to 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 upon the actions of tens of thousands of individual parents, while 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. For example, it is possible that a program with only nighttime and passenger restrictions would have been effective, but there were too few State-quarters when such programs were in effect to test this hypothesis. 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 age 20 to 29 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 al.28 indicated that the presence of adults age 20 to 29 was associated with severe or fatal injury among 16- and 17-year-old drivers even when alcohol use was controlled. Williams and Shabanova 29 reported that teen drivers were less likely to use safety belts when passengers were in their twenties, and recommended that passenger restrictions not be waived unless there is a supervisor at least 30 years old. This recommendation is reasonable because mature passengers are more likely to take some responsibility for the safety of a trip.

Finally, our findings may underestimate the benefit of GDL because we assumed that a restriction imposed at any time during a three-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 following the effective date of each GDL requirement.