Research Brief

Behavioral Impact of Graduated Driver Licensing on Teenage Driving Risk and Exposure

By Pinar Karaca-Mandic, PhD
Assistant Professor
Division of Health Policy and Management
School of Public Health
University of Minnesota

Greg Ridgeway, PhD
Senior Statistician,
RAND Corporation

Motor vehicle crashes are the leading cause of death among teenagers. In 2005, a total of 5,300 teenagers, ages 13 to 19, died in a motor vehicle crash, representing 33 percent of all deaths among this age group. Various studies have attributed this increased risk level to factors such as a lack of driving experience, tendendacy to drive at riskier times (night time), and fellow teenage passengers that lead to a distracting environment for the driver.

Over the last 12 years, almost all U.S. states have adopted graduated driver licensing (GDL) policies, which have been previously implemented in Australia, New Zealand, and several Canadian provinces as tools to reduce traffic hazards for teenagers. These policies introduce three distinct licensing stages: learner’s permit, intermediate license, and full licensure. The first two stages typically involve requirements on the minimum number of hours of supervised driving, as well as driving restrictions especially in high-risk situations (night-time and with teen passengers). These restrictions are gradually lifted as teen drivers mature and advance to the next stage. The final stage, “full licensure”, removes all the restrictions. The purposes of these policies include expanding the learning process, reducing risk exposure, improving driving proficiency and encouraging safe driving.

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Previous literature evaluating the impact of GDL policies of the 1990s has demonstrated the policies’ effectiveness on reducing teenage-involved fatal crashes. However, the mechanisms through which these reductions are achieved are unexplored. In particular, does GDL help improve teen driving, or does it simply limit the number of teens on the road especially in high-risk situations? The difficulty in investigating this issue is due to the unavailability of good estimates of how many teenage drivers are on the roads at any given time, and how that varies with the GDL policies. (In a sports analogy, it is like trying to guess a baseball player’s batting average using only the number of hits.) Moreover, it is unknown whether GDL policies have long-term effects on teenage driving. For example, do GDL exposed teens become better drivers in the future? Alternatively, do GDL regulations shift risky driving to older teens by disallowing them to mature through risky behavior while they are younger? Our goal is to investigate such behavioral implications of the GDL policies.

Study Design

We estimate a structural model that separately identifies GDL’s effect on the relative teenage prevalence and relative teenage riskiness. We also distinguish between states that implement stricter GDL policies. Our ability to identify the relative crash risks and relative exposure relies on data from two-car crashes and the information contained in the counts of teen/teen, teen/adult, and adult/adult crashes. There are only certain relative risks and exposures that would make a particular collection of crash counts plausible. If many of the accidents involve teen/teen collisions then either the teen-relative risk is high or there are many teens on the road relative to adults. The counts of teen/adult and adult/adult crashes can be used to identify how much the number of teen/teen crashes depends on risk and how much depends on exposure.

We model these crash frequencies as a function of the teenage driving exposure and driving risk, both relative to those of adults. Next the relative amount of teenage driving and teenage driving risk are related to the GDL policies as well as other state-year level driving related laws and demographic information. The model is estimated jointly in a two-level hierarchical modeling framework.

We use a relatively new dataset called the State Data System (SDS) which has data from 1990 to2005.   This data is very similar to the Fatal Analysis Reporting System (FARS) data used extensively by previous research on traffic safety. While FARS reports only fatal accidents, SDS includes all police reported crashes and provides information on all persons involved in the crash (age, gender, drinking, status, etc.) as well as detailed information on the crash (location, time, road type, road conditions, number of vehicles involved, etc. ) and on all the vehicles involved (make, model, year). Unfortunately, not all states report to SDS. We obtained data from 12 states

Findings

We find that the GDL policies have reduced the number of accidents by 15 to 17 year-olds by limiting the amount of teenage driving rather than by improving teenage driving. More specifically, GDL reduces relative teen prevalence by 5 percent during the day and 15 percent during the night. Stricter GDL policies, especially those with night-time restrictions have been significantly more effective in limiting teenage driving at night. For instance, a GDL policy rated “good” (i.e. most strict) by the Insurance Institute of Highway Safety reduces teen prevalence at night time by 43 perecent while a GDL policy rated “fair” reduces it by 12 percent. We do not find statistically significant effect of the policies rated “marginal” (i.e. least strict) on relative teen prevalence.

We investigate whether driving under GDL restrictions results in a cohort of drivers that have reduced risks in the future, although the effects during the early teen years might be insignificant. Although we do not find evidence that GDL policies make teenagers become better drivers in later years, future research should revisit this issue as more teens graduate from the GDL policies.

From a public health perspective, our findings suggest that more restrictive GDL policies for 15 to 17 year-old drivers reduce teen accidents and fatalities. Stronger GDL policies defer unrestricted driving, thus reducing teens’ exposure to high risk driving situations. Recent neurological studies may help explain our findings. In particular, National Institute of Mental Health researchers found that regions of the brain governing impulse control, prioritization, and strategy (the dorsal lateral prefrontal cortex) are still “under construction” during teen years and do not develop fully until early 20s (Giedd, 2004). Ultimately, biological or behavioral maturation, rather than experience, may be the mechanism through which GDL reduces teens’ driving accident risk.  Naturally, further research may provide insight into optimal GDL policy structure, as well as how alternative policies might influence learning by teen drivers.

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