California Partners for Advanced Transportation Technology

In July 1998 California changed its graduated driver licensing laws (GDL) for new drivers under the age of 18 to include restrictions on hours of driving, carrying teen-age passengers, and requiring more adult supervised driving practice. With fatal and injury crash data from California's Statewide Integrated Traffic Records System, this study, sponsored by Caltrans, used standard regression analysis as well as the Bai-Perron stochastic multiple structural break model to determine the effect of the law on teen-age passengers and crash rates of 16 year-old drivers. We found that in the four and one-half years following implementation of the new law, crashes caused by 16 year-old drivers decreased by 11% and the average number of teen-age passengers carried by 16 year-olds decreased by approximately 31%. The combination of these two decreases resulted in the saving of 29 lives and the prevention of 2,632 injuries.

To test the specific effect of the restrictions on driving between midnight and 5 AM, regression analysis was performed on the quarterly percentage of curfew crashes for 16 year-old drivers. Quarterly data was used due to the relatively small number of curfew crashes. The percentage of curfew crashes have been in a nearly significant long term down trend since 1996. There was a small, non-significant lessening of the long-term downward trend at the time of the implementation of the new law. The new law does not appear to have had a material effect on the percentage of driving done by 16 year-olds during curfew hours. This percentage was in a downtrend prior to the law and continued after the law took effect.