IMPORTANCE: Many studies have failed to show an effect of parent-supervised practice driving on the driving performance of teenagers; nevertheless, most Graduated Driver Licensing programs have provisions that require supervised practice. OBJECTIVE: To determine whether a web-based intervention, the Teen Driving Plan (TDP), can improve the driving performance of teenagers before licensure as measured by the Teen On-road Driving Assessment (tODA). DESIGN, SETTING, AND PARTICIPANTS: Randomized, single-blind, clinical trial among 217 dyads (1 parent: 1 teenaged learner's-permit holder) to test TDP effectiveness on increasing the quantity and diversity of supervised practice and improving the teenagers' prelicensed driving performance. The study was conducted from December 2011 through January 2013 in Southeastern Pennsylvania. INTERVENTIONS: Dyads were randomized (3:2) to receive the TDP or the Pennsylvania driver's manual (control group). The TDP is a psychoeducational intervention designed to increase the quantity and diversity of parent-supervised practice. Materials are grouped by the following driving environments: empty parking lots, suburban residential streets, intermediate (1- or 2-lane) roads, highways, rural roads with curves and elevation changes, and commercial districts. MAIN OUTCOMES AND MEASURES: The main outcomes were self-reported practice driving across 6 environments and 2 conditions and driving performance as measured by the teenagers' completion of the standardized and validated tODA 24 weeks after enrollment. Certified professional driving evaluators blinded to randomization status terminated the tODA if they determined that the teenager could not safely complete it. We examined mean differences in the quantity of supervised practice, differences in the overall proportion of teenagers in each group that had assessments terminated for unsafe driving, and the point of termination during the assessment. RESULTS: The TDP dyads reported more practice in 5 of the 6 environments and at night and in bad weather compared with the control dyads. Overall, 5 of 86 TDP teenagers (6%) had the tODA terminated compared with 10 of 65 control teenagers (15%) (risk difference [TDP – control], -9% [95% CI, -21% to 2%]; P = .06). The hazard ratio for exposure to TDP was 0.35 (95% CI, 0.12-1.03; P = .05, log-rank test). CONCLUSIONS AND RELEVANCE: Preliminary evidence suggests that the TDP improves supervised practice and the driving performance of prelicensed teenaged drivers. Future studies can explore how to revise the TDP to enhance the treatment effect and how best to disseminate the TDP without compromising implementation fidelity. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01498575.