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Status Report, Vol. 48, No. 9 | December 19, 2013Subscribe

Study of teen fatal crash rates adds to evidence of GDL benefits

A recent study adds to the evidence that graduated driver licensing (GDL) systems are working to cut fatal crashes among 16 and 17 year-olds. Researchers analyzed specific GDL components and found that permit holding periods of nine months to a year and a one-passenger limit during the intermediate license stage had the biggest benefits.

Researchers from the California Department of Motor Vehicles and the University of North Carolina used fatal crash data and population data from 1986 to 2007 to analyze the effects of various components of GDL laws across the nation.

They found that fatal crash rates for 16-17 year-olds were 21 percent lower with permit holding periods of nine to 12 months, compared with no holding period. A limit of no more than one passenger was associated with a 15 percent reduction in fatal crash rates, compared with no passenger restriction. Two other provisions — an intermediate license age of 16½ to 17 and a nighttime restriction of 10 p.m. or earlier — were associated with fatal crash rate reductions for 16 year-olds but had no significant effect on crash rates of 17 year-olds.

Based on earlier research by IIHS and HLDI, the Institute estimated in 2012 that if every state adopted all five components of the toughest GDL laws in the nation, more than 500 lives could be saved and more than 9,500 collisions could be prevented each year (see "How to make young driver laws even better," May 31, 2012). A calculator allows users to see how adjusting any of the five provisions — permit age, practice hours, license age, night driving and passenger limits — could affect collision insurance claim rates and fatal crash rates among 15-17 year-olds in a given state.

In the latest study, the researchers found that minimum learner permit holding periods reduced fatal crash rates if they lasted at least five months, but holding periods of nine to 12 months were associated with much bigger reductions. The holding period may help by increasing the time the teenager is driving with supervision and providing young drivers with more practice time, the authors suggest. The IIHS study found no additional benefit from a holding period, once practice hours and the effect of the holding period on licensing age were taken into account.

When it comes to passenger restrictions, the study found that a limit of one teen passenger resulted in a greater reduction of fatal crash risk than complete bans on passengers. The authors hypothesize that young drivers are more likely to comply with a one-passenger limit than an outright ban. However, in the IIHS study, total bans on passengers were found to be more effective than one-passenger limits.

The study also found that a licensing age of 16½ or 17 resulted in the lowest fatal crash rates for 16 year-olds, likely because it resulted in fewer 16 year-olds driving unsupervised (or very few in the case of 17). A night driving restriction of 10 p.m. or earlier reduced fatal crash rates of 16 year-olds by 19 percent.

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