Effects of Graduated Driver Licensing on Fatalities in 16-Year-Olds

Abstract

To the Editor.—Motor vehicle crashes are the most common cause of death among youth populations in developed countries in the world.1 Among initiatives developed to reduce motor vehicle crash–related injuries, deaths, and traffic convictions among youth, graduated driver licensing (GDL) laws have demonstrated effectiveness. In particular, a recent retrospective study by Chen et al2 indicated that GDL program components leading to reduced motor vehicle crash fatalities were GDL programs that include age requirements and ≥3 months of waiting before the intermediate stage, nighttime driving restriction, and either ≥30 hours of supervised driving or passenger restriction.Although Chen et al2 provided a valuable contribution to the evidence base, they seem to have overlooked important variables that could affect their outcome (motor vehicle crash fatality), including vehicle miles traveled, vehicle density (number of registered motor vehicles divided by length of roads), speed-limit regulations, alcohol-impaired–driving laws (zero tolerance laws, minimum drinking-age laws), and economic factors (rate of unemployment, real per-capita income, etc). When a relevant variable is omitted from a regression model, the resulting estimators of effect in the final model are biased because they share the variance that would have been attributed to the excluded variable. All factors need to be controlled for in a study of traffic crashes, and results of a study concentrating on a single program (ie, GDL) are possibly misleading. GDL programs cannot be considered to be effective without accounting for confounding in statistical analysis by other laws or programs that also seek to reduce motor vehicle crashes among youth.Driving laws related to alcohol impairment are a particularly important confounder that could also be responsible for reductions in motor vehicle crash injuries and fatalities among youth. Zero tolerance laws, which make it illegal in every state for drivers younger than 21 to drive after any drinking, have significantly contributed to declines in alcohol-related traffic fatalities among people younger than 21. In 1995, the US Congress passed a law that requires states to adopt zero tolerance laws for drivers younger than 21. By 1998, all states had passed laws that made it illegal for any driver younger than 21 to have a positive blood alcohol content (BAC). Voas et al3 used data on all drivers younger than 21 involved in fatal crashes in the United States from 1982 through 1997. Quarterly ratios of BAC-positive to BAC-negative drivers in each of the 50 states were analyzed in a pooled cross-sectional time-series approach. After accounting for differences among the 50 states in various background factors, changes in economic and demographic factors within states over time, and the effects of other related laws, results indicated a significant 24.4% reduction in alcohol-positive drivers younger than 21 who were involved in fatal crashes associated with the zero tolerance laws. Similar results were shown by another systematic review4; therefore, this policy seems to have been significantly effective in reducing fatal crashes involving youth drivers.5,6In addition to zero tolerance laws, other drinking-and-driving laws were also passed in the United States between 1995 and 2005. Many states now revoke or suspend the licenses of drunken-driving offenders who are under the legal age. Characteristics of crashes that take place with alcohol-impaired youth drivers include speeding disproportionately, lack of restraint use, and driving older cars that are in poor repair. Primary and secondary seat belt–enforcement laws, increased speeding restrictions, and improved safety standards for vehicles and highway construction may all help reduce fatal crashes among young drivers. For instance, citizen and community-based advocacy groups have played important prevention roles in areas such as youth drinking and driving and enhancement of youth restraint use.7Further understanding of the effect of these factors in combination with GDL laws should help in the study and implementation of best practices to reduce youth-involved traffic crashes worldwide. Methodologically sound research is needed to evaluate GDL systems and identify which GDL elements produce the greatest crash reductions. To the extent possible, researchers should seek to use appropriate statistical methods that facilitate comparisons across jurisdictions, acknowledging the considerable variation in GDL laws and effects of non-GDL laws and systems across different jurisdictions.