Teenage drivers—both males and females—were more likely to tailgate and exceed the speed limit if there was a teenage male passenger in the front seat, according to a study by the National Institute of Child Health and Human Development of the National Institutes of Health.

Conversely, male teenagers were less likely to tailgate or exceed the speed limit when a teenage female was in the front passenger seat.

In addition, female teen drivers were slightly more likely to tailgate if there was a female teen passenger in the vehicle with them.

The study was published on-line in Accident Analysis and Prevention and will appear in a future edition of that journal.

"This study provides information that will be useful for officials in devising teen licensing standards," said NICHD Director Duane Alexander, M.D. "The findings indicate that teen risky driving increases in the presence of teen passengers, particularly male teen passengers. But more important, the finding should remind teens—and the adults who care about them—that they need to drive safely, regardless of who is in the passenger seat."

The study was unable to determine why the presence of teen males increased the likelihood of speeding and tailgating, said the study's first author Bruce G. Simons-Morton, Ed.D., M.P.H, Chief of NICHD's Prevention Research Branch.

Crash rates for 16- and 17-year-old drivers are higher in the presence of teen passengers, Dr. Simons-Morton and his colleagues wrote. However, researchers do not understand the reasons for these higher crash rates. Dr. Simons-Morton and colleagues at the survey research firm Westat undertook the current study to learn how the presence of teen passengers might affect teens' driving behavior.

To conduct the study, the researchers positioned observers at the parking lot exits of 10 high schools in the suburban Washington, D.C. area. The observers took notes on the make and model of the departing vehicles, as well as the age and gender of the driver and passengers. A second group of observers was stationed ½ to ¾ of a mile away from the parking lot, and used video recording equipment and a laser-assisted radar device to measure traffic flow. This second set of observers charted the speed of the vehicles and measured vehicle headway, an indication of how closely vehicles follow the vehicles in front of them. The study authors defined vehicle headway as the time (in seconds) between vehicles as they passed a fixed point in the roadway.

More than 3000 passing vehicles were recorded at the second site. Of these, 2251 were vehicles in general traffic, and 471 were teen drivers (245 male and 226 female). No passengers were present in 232 of the teen vehicles, and one or more passengers were present in 239 of the teen vehicles.

On average, teens drove 1.3 miles an hour faster than the general traffic. Moreover, the average headway for teen drivers was about .17 seconds shorter than for the general traffic (about 10 feet less at 40 miles an hour).

Both male and female teenage drivers were most likely to drive faster than the general traffic and to allow shorter headways if there was a male teenage passenger in the car. In fact, when a male passenger was in the vehicle, a quarter of teenage drivers exceeded the speed limit by at least 15 miles an hour.

(A graph showing the percentage of teenagers driving over 15 miles an hour is posted at http://www.nichd.nih.gov/new/releases/teenage_drivers_statistics.cfm.)

Similarly, both male and female teens drove faster and allowed shorter headways in the presence of a male teenage passenger when compared to teens who had either no passengers or a female teen passenger. However, teenage males allowed longer headways in the presence of female passengers.

On average, headways were .3 seconds shorter for male teens drivers with male teen passengers, and .15 seconds shorter for female teen drivers with female teen passengers.

"At typical driving speeds of around 40 mph, a 0.3 [seconds] difference is equivalent to traveling slightly more than one car length closer to the vehicle ahead," the authors wrote.

In the article, the study authors explained that although they studied vehicle headway and speed independently, these two factors are probably related. "Close following headways may constrain speed; fast driving may result in close following," they wrote.

For this reason, the authors charted the proportion of teens engaging in some form of risky driving, which they defined as either driving with a headway of less than 1 second, and speeds 15 or more miles above the posted speed limit.

According to these criteria, of the 14.9 percent of teen males engaging in risky driving, 21.7 percent had a male teen passenger in the vehicle. In contrast, only 5.5 percent of teen male drivers showed risky driving behavior in the presence of a female passenger.

Of the 13.1 percent of teen female drivers showing risky driving behavior, 12.9 percent had a male teen passenger, and 15.5 percent had a female passenger. Dr. Simons-Morton said that most cases of risky driving in this 15.5 percent of risky teen female drivers were due to short headways.

Dr. Simons-Morton noted that the current study could not identify why teens were more likely to engage in more risky driving behavior in the presence of teen passengers. Teen passengers may distract the driver or change the driver's attitude or emotion in ways that are not yet clear. To find answers, he and his colleagues are currently designing a study that will involve placing electronic monitoring equipment in vehicles with teen drivers. After learning the reasons for the risky behavior, researchers can then work to develop ways to prevent it.

Until answers become available, Dr. Simons-Morton cautioned parents and teens to be aware of a tendency that teens appear to have toward risky driving when other teens are in the vehicle with them, and to be extra vigilant against unsafe driving under these conditions.

The NICHD is part of the National Institutes of Health (NIH), the biomedical research arm of the federal government. NIH is an agency of the U.S. Department of Health and Human Services. The NICHD sponsors research on development, before and after birth; maternal, child, and family health; reproductive biology and population issues; and medical rehabilitation.