Since we live on a globe, all those angles are bigger than 90°, making it only a quadrilateral.

Legality aside, how are headphones worse than radio? If you can hear sirens, car horns and whatever else you need, why does it matter if the music comes from a radio speaker or headphones? Keep in mind, some countries use the same reasoning for cars and bikes, and I’d rather not annoy everyone with drive-by beats.

Also, I imagine headphones in cars is more common if you are young and your car is old.