Angelina Jolie, Brad Pitt's Son Pax Jolie-Pitt Injured in Bicycle/Car Accident

Angelina Jolie and Brad Pitt 's son, Pax Jolie-Pitt , is one lucky guy after police say he smashed his electric bicycle into a car on a busy L.A. street ... TMZ has learned.

The 20-year-old was riding Monday on Los Feliz Blvd. around 5 PM -- so, rush hour on a heavily trafficked street -- and as he approached a red light, he rammed into the back of a car stopped at the intersection ... according to law enforcement sources.

We're told the driver of the car got out to check on Pax, who was not wearing a helmet at the time of the accident. Police and paramedics arrived on the scene, and witnesses tell us Pax was experiencing hip pain, and had apparently suffered a head injury ... so, he was rushed to an L.A. hospital for treatment.

We're told medics feared he suffered a minor brain bleed in the wreck, but the latest word is that Pax is stable, and might be able to go home sometime tonight.

Pax -- the 4th of Angelina and Brad's 6 kids -- has been seen recently cruising around L.A. on a BMX-style e-bike, and almost always without a helmet.

California helmet laws differ depending on the class of bicycle ... so, it's unclear if he needs one with that particular ride. What is clear, is a helmet would've come in handy during today's accident.

Pax has dabbled in acting ... landing a voice over role in "Kung Fu Panda 3" and uncredited role in his mom's Disney flick, "Maleficent."

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We've reached to Angelina's reps for comment, but no word back yet.