On a warm Sunday afternoon last March, Shelby, 18, was driving three of her closest friends back home from South Padre Island, a popular resort town located just off Texas’s southernmost tip. It was the end of their spring break vacation, and the girls were soaking up every last second together before returning home. 

Maybe they had the music turned up high, singing along. 

Maybe Jade, 17, was talking to her older sister, Brianna, about how psyched she was to play college volleyball next year. 

Or maybe all three of the passengers, exhausted from all the sun and staying up late, were fast asleep while Shelby navigated her way back to the Houston suburbs alone. 

Sadly, we may never know what these friends were doing as they rolled along Highway 77 near Corpus Christi, Texas, but what police were able to piece together is that Shelby took her eyes off the road, for just a moment, to check her phone’s GPS. 

And that seemingly harmless split-second decision landed her in critical condition—while killing all three of her friends. 

Driving is possibly the ultimate rite of passage, and it doesn’t matter whether you’re finally able to borrow the car yourself, or you rely on your older friends for a ride. There’s nothing like hopping in the car to determine your own fate: the playlist you pick to sing along to, the totally spontaneous decision to grab a snack instead of seeing a movie.

With all this freedom at your fingertips, it’s easy to push the risks to the back of your mind. But experts say that mind-set can be deadly, because what happened to Shelby and her friends is not uncommon. Car crashes are the leading cause of death for U.S. teens, and ages 16 and 17 are the deadliest two years in a person’s entire life.

That’s not only because you’re still mastering the skill of driving, but also because you’re struggling with the sometimes complicated responsibility of being a good passenger. Just being aware of your limits as a driver and your power from the backseat, however, can save lives.

“Please don’t call car crashes accidents,” says Jayce Good, a safe-driving advocate with a personal connection to the cause. As she was returning home from her college graduation in 2008, a teen talking on his phone turned left through a red light, causing a fully loaded tractor-trailer to hit her family’s station wagon head on—killing her parents and leaving Jayce with a permanent brain injury. “Ninety-three percent of car crashes are caused by human error, so we really do have the power to keep people alive.” 

While distracted and drowsy driving are crucial parts of the equation explaining teens’ risks on the road, perhaps the most interesting pattern researchers have found is related to passengers. For 16- and 17-year-olds, the risk of being killed in a crash increases by 44 percent when one passenger younger than 21 is in the vehicle. With two passengers, it doubles. And with three or more? That risk quadruples. What do you, sitting in the backseat, have to do with it?

First, for a new driver—who is still learning to detect hazards—having friends in the car presents the most intense set of distractions. But it also has to do with feeling emboldened to test the limits of your newfound independence by showing off. According to one study, male teen drivers were almost six times more likely to perform an illegal maneuver and twice as likely to drive aggressively before crashing if they were carrying passengers.

That’s why, across the board, driving experts are calling for a new movement among young people, where they speak up if they feel unsafe—and take their co-pilot role as seriously as they would a position behind the wheel.

“As a passenger, you have a tremendously important job in the car,” says Klauer. Not only should you act as the designated texter and DJ to make sure your friend is able to maintain his or her concentration, but you should also be helping the driver spot potential hazards on the road. “We like to say that a passenger can act as ‘a collision avoidance warning system’ for things that the driver has missed,” Klauer continues. It’s a daunting, but crucial, job.

Just think about it: Maybe if someone else in the car had been navigating for Shelby that day, looking up directions or watching for oncoming traffic, she might not have swerved into that hulking 18-wheeler. Maybe all three of her friends would still be alive.