It’s a fairly common sight, particularly on highways to see someone in the passenger seat of a car have their feet up on the dashboard. Few worry about the consequences of being involved in one of the 3,400 car accidents that happen daily, but it’s actually the device designed to keep you safe that can (literally) destroy your legs.

Here’s why you should never put your feet up on the car’s dashboard, if you’re not entirely convinced already.

Airbags Faster Than You

Car airbags are designed to deploy in accidents as slow as 12kph (8mph) into a solid barrier. The average car airbag inflates completely within 1/20th of a second, about twice the speed of an average eye blink. An airbag needs to get in place (and even begin partially deflating) before your head collides with the windshield or dashboard. Airbags are faster than a human blinks, let alone physical reactions.

So it’s impossible to move your legs out of the way in time during an accident.

Achieving these speeds requires a lot of force (roughly 3.33×10⁴ Newtons) – one good reason you should never put your phone or any other object over the airbag ever. When your airbag deploys it can turn an air freshener or your legs into a powerful projectile.

Knees Coming At You

Overestimating how fast you can react to move your legs out of the way is something many passengers do, as is underestimating what an airbag does to your legs. Numbers and Newtons are hard to grasp but the Mythbusters video below does a good job of showing you how bad things can look.

Part of the issue might be the name “air” bag. We tend to associate air with something soft. Bags are those fluffy things that hold our groceries but Audra Tatum’s story might change your mind on both counts.

Adding Injury To Injury

Make no mistake about it – when an airbag deploys if your legs are in the way they’ll be forcibly displaced. Some people have their faces damaged or eyes lost because your knees come hurdling back under the strong force of the airbag. Others, like Tatum, suffer severe compound leg breaks, as you can read below (or click on the link to see a photo which should be warning enough).

“The airbag went off, throwing my foot up and breaking my nose,” Tatum, who wasn’t wearing a seat belt, told CBS. “I was looking at the bottom of my foot facing up at me.” Besides her nose, Tatum broke her ankle, femur and arm, telling CBS she still walks with a limp and can’t stand more than 4 hours at a time.

Another thing to note is airbags don’t always deploy due to a collision, millions are recalled each year due to malfunctions like activating when they’re not supposed to. I won’t bother telling you stories of people who had their legs out the window before an accident. Needless (I hope) now to say, don’t put your feet up on the car dashboard, they’re hardly as safe as planes, and rather let airbags do what they’re good at. For those of you who travel with pets, don’t let them hang out in the front seat or better yet, make sure they’re secured in a proper car harness in the back.