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Status Report, Vol. 41, No. 7 | September 7, 2006 Subscribe

California GDL cuts crashes, especially in high-risk situations

A new Institute study quantifies the benefits of provisions of California's graduated driver licensing program. Findings affirm those of previous studies by other researchers that also point to the success of this program.

California legislators were among the first in the nation to enact graduated licensing, which took effect in 1998. The law increased the learner's permit period and required parents or guardians to certify that learners get at least 50 hours of practice. Once licensed, 16 year-olds still are restricted. They may not drive unsupervised at night or any time with teen passengers.

Prompted by another study in 2003 that found no effects of these provisions, Institute researchers conducted their own evaluations. The main finding is a 23 percent overall reduction in the per-capita crash involvement rate of 16-year-old drivers. Crashes went down more in the high-risk situations specifically addressed by graduated licensing. Nighttime crash rates went down 27 percent, and crash rates with teen passengers decreased 38 percent.

Benefits of this law aren't being achieved by postponing the crash problem until 16 year-olds get a little older. That is, the law isn't shifting high crash rates from 16-year-old beginners to 17 year-olds, whose crashes also declined after graduated licensing.

Nor are crashes going up among beginners driving alone, despite concern that the passenger restriction would increase the risk by forcing groups of teens to travel in separate vehicles to the same destination. The Institute found reductions in crashes in which beginners drove alone, went as passengers, etc.

"To affirm these findings, we conducted our analyses several ways, all of which revealed positive results. So we know the law is successful," says Susan Ferguson, Institute senior vice president for research.

Multiple studies of the graduated licensing laws in other states also reveal benefits, including fatal crash reductions (see "Young drivers' crash rates decline sharply under graduated licensing," Feb. 17, 2001). An exception is a study published in 2003 by the California transportation department, which found no overall effects of graduated licensing.

This result is out of line not only with findings in other jurisdictions but even with results of two other evaluations of the California law that were conducted before the Institute's analyses. One of these studied three years of data, concluding that graduated licensing reduced the per-capita crash rate of 16-year-old drivers by 28 percent. Other researchers reported a 17 percent reduction, based on comparison of data during 21 months before the California law took effect and an equal period afterward. Both studies were published in 2004.

The Institute's study relies on the same data sources as all of the previous studies. Institute researchers used time series to account for trends before graduated licensing took effect. Crashes of 24-55 year-olds also were considered to account for fluctuations in driving conditions, seasonal effects, and other factors that might have been influencing crash rates during the period of study, 1995-2003.

"Our analyses weren't all that different from the approach used by the researchers at the California transportation department, who found no effects of graduated licensing. In fact, there isn't anything inherently wrong with what they did. They used time series, just as we did. It's a routine statistical method, but in this case the California researchers didn't use time series to factor in all of the various trends in crashes of 16 year-olds that were occurring before the graduated licensing law took effect. For example, they didn't take into account some of the seasonal variations in the data," Ferguson explains. "Once we took these other factors into account, the benefits of graduated licensing were clearly revealed."

California may be reaping even greater benefits since the Institute's study was conducted because the law itself has been strengthened. The new law has raised the minimum age for a learner's permit to 15-1/2 from 15, started the nighttime restriction an hour earlier (11 p.m. instead of midnight), and extended passenger restrictions to cover the first year of licensure instead of the first 6 months. These provisions make California's one of the strongest graduated licensing laws in the United States.

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