From JAMA: Graduated drivers licenses and fatal crashes in teen drivers

http://www.ncbi.nlm.nih.gov/pubmed/21917580

JAMA. 2011 Sep 14;306(10):1098-103.
Graduated driver licensing and fatal crashes involving 16- to 19-year-old drivers.
Masten SV, Foss RD, Marshall SW.
Source
California Department of Motor Vehicles, Research and Development Branch, 2570 24th St/MS H-126, Sacramento, CA 95818-2606, USA. scott.masten@dmv.ca.gov

Abstract
CONTEXT:
In the United States, graduated driver licensing (GDL) systems allow full, unrestricted licensure for drivers younger than 18 years only after an initial period of supervised driving and an intermediate period of unsupervised driving that limits driving at night, transporting multiple young passengers, or both.

OBJECTIVE:
To estimate the association of GDL programs with involvement in fatal crashes among 16- to 19-year-old drivers.

DESIGN, SETTING, AND PARTICIPANTS:
Pooled cross-sectional time series analysis of quarterly 1986-2007 incidence of fatal crashes involving drivers aged 16 to 19 years for all 50 states and the District of Columbia combined.

MAIN OUTCOME MEASURES:
Population-based rates of fatal crash involvement for 16-, 17-, 18-, and 19-year-old drivers. Rate ratios and 95% CIs comparing state-quarters with stronger (restrictions on both nighttime driving and allowed passengers) or weaker (restrictions on either nighttime driving or allowed passengers) GDL programs with state-quarters without GDL.

RESULTS:
Fatal crash incidence among teen drivers increased with age, from 28.2 per 100,000 person-years (16-year-old drivers) to 36.9 per 100,000 (17-year-olds), before reaching a plateau of 46.2 per 100,000 (18-year-olds) and 44.0 per 100,000 (19-year-olds). After adjusting for potential confounders, stronger GDL programs were associated with lower incidence of fatal crashes for 16-year-old drivers, compared with programs having none of the key GDL elements (rate ratio, 0.74 [95% CI, 0.65-0.84]). However, stronger GDL programs were associated with higher fatal crash incidence for 18-year-old drivers (rate ratio, 1.12 [95% CI, 1.01-1.23]). Rate ratios for 17-year-olds (0.91 [95% CI, 0.83-1.01]), 19-year-olds (1.05 [95% CI, 0.98-1.13]), and 16- to 19-year-olds combined (0.97 [95% CI, 0.92-1.03]) were not statistically different from the null.

CONCLUSIONS:
In the United States, stronger GDL programs with restrictions on nighttime driving as well as allowed passengers, relative to programs with none of the key GDL elements, were associated with substantially lower fatal crash incidence for 16-year-old drivers but somewhat higher fatal crash incidence for 18-year-old drivers. Future studies should seek to determine what accounts for the increase among 18-year-old drivers and whether refinements in GDL programs can reduce this association.