As the number of passengers increases in a car driven by a teenage driver, so does the likelihood of fatal injury to the driver, according to researchers at the schools of Public Health and Medicine and the Insurance Institute for Highway Safety. The study supported the basic premise of graduated licensing systems, which allow teenagers to earn full driving privileges step by step through increasing the period of supervised driving and restricting unsupervised driving at night or with passengers. The results of the study were published in the March 22 issue of the Journal of the American Medical Association.

Statistics show motor vehicle crashes are the leading cause of death among teenagers in the United States. The purpose of the present study was to determine whether the presence of passengers made a difference in the risk of crashes fatal to teen drivers. "Knowing the circumstances associated with increased risk to teenage drivers is useful in formulating graduated driver licensing programs and in advising health professionals who take care of teenagers," said lead author Li-Hui Chen, a research associate in the School of Public Health. "Our study examined the relationship between crashes fatal to 16- and 17-year-old drivers and the characteristics of passengers."

The study looked at several potential risk factors, including the age and sex of the driver, the number of passengers, passenger age and sex, and the time of day. The data originated from three federal sources: the Fatality Analysis Reporting System, the Nationwide Personal Transportation Survey and General Estimates Systems.

The researchers found that drivers aged 16 and 17 years had a much higher risk of dying in a crash than did older drivers and that, compared with driving alone, driver deaths per 10 million trips increased with the number of passengers. The highest death rate was among 16-year-old drivers carrying three or more passengers (5.61 per 10 million trips). In contrast, death rates for drivers aged 30 to 59 years were actually lower with passengers than without. In all cases, male drivers had a higher death rate than female drivers regardless of the number of passengers. Driver deaths per 1,000 crashes increased for 16- and 17-year-old drivers with male passengers or passengers younger than 30 years.

The implementation of graduated driver licensing by New Zealand and Canada has reduced the number of teenage driver crashes in those countries. Current graduated licensing systems in the United States either don't have passenger restrictions or only apply them for a short time. Results of the study show that restrictions on carrying passengers should be considered for inclusion in graduated licensing systems for young drivers. The authors stressed that parents of teenage drivers should be made aware of the increased danger associated with transporting passengers.