EMERGENCY room doctors are warning Americans about a simple but dangerous mistake that can turn a routine car ride into a life-changing trip to the hospital.

Across the US, hospitals see millions of patients every year - and many arrive after car crashes that could have been prevented.

Doctors say one everyday habit, seen on highways in California to Nevada, is leading to some of the most severe injuries they treat.

It's a position many passengers find comfortable, until disaster strikes.

Medical experts are now begging people to stop putting their feet on the dashboard.

Dr. Solomon Behar, a pediatric emergency medicine physician at Miller Children's & Women's Hospital and MemorialCare Long Beach Medical Center in Long Beach, California, said the habit makes him "cringe" every time he sees it, Parade reported.

"I do a lot of driving on Los Angeles freeways, and one of the things that makes me cringe is when I pass a passenger who has their feet up on the dashboard," Dr. Behar said.

He explained that during a crash, the combination of the airbag force and the passenger's forward motion can cause devastating damage.

"The force of the airbag and the forward momentum of the passenger can force the knees and legs into the dashboard, and you can get really severe, life-threatening injuries such as fractures, dislocations and ligamentous injuries to the pelvis, knees, and the hips," he said.

Dr. Jacob Snow, a pediatric emergency room specialist at Pediatrix Medical Group in Las Vegas, Nevada, issued the same warning.

"There is always some risk of being involved in a car accident every time you ride or drive," he said.

"Riding with your feet on the dash adds unnecessary risk."

Dr. Snow explained that even a sudden stop, not just a major crash, can lead to broken bones or dislocated joints.

"Your body will move forward with the momentum of the car and that can lead to fractures of the leg bones or dislocated joints," he said.

He added that airbags, which inflate in a fraction of a second, can make the injuries even worse.

"These airbags are intended to protect the passenger; however, if your legs are on the dashboard, the force of the airbag can severely injure the legs or push your legs back into your face," he said.

Some injuries can have lasting effects, especially for younger passengers.

"If a child sustains a serious hip injury or fracture and damages the growth plate, it can lead to early arthritis and potentially end with the child needing a hip replacement early in adulthood," Dr. Snow said.

Both doctors agree the safest position in a car is simple - feet flat on the floor, knees bent, and seatbelt on.

Sitting properly allows the car's safety systems to do their job.

Modern vehicles are built with passenger's safety in mind.

Many vehicles have special airbags near the legs that are meant to shield passengers in crashes but they only work if you are sitting correctly.

Crossing your legs, reclining too far, or lying on your side can also increase your risk of serious injury.

Urgent warning for US drivers as ER doctor reveals the most common crash injury