Ambulances have the word Ambulance flipped backwards, not flipped upside down.
This intuitively seems to be a major break of symmetry, why would a mirror have a horizontal preference?
It isn't the mirror with the preference, but the vehicle. To understand why, consider an ambulance facing you head on with the word "AMBULANCE" printed on it and the paths it could take to show up in your rear-view. It could move along a lateral half-circle or a vertical one (or something in-between). If it went along a vertical half-circle it would end up upside-down and behind you; since it goes along a lateral one it also gets laterally-flipped, but because cars have left to right mirror symmetry , you don't notice (except for the words ambulance).
We have a huge bias evolutionary bias towards that type of lateral symmetry (e.g. in animals, other humans) and to things moving along a lateral plane while maintaining vertical orientation.
This intuitively seems to be a major break of symmetry, why would a mirror have a horizontal preference?
It isn't the mirror with the preference, but the vehicle. To understand why, consider an ambulance facing you head on with the word "AMBULANCE" printed on it and the paths it could take to show up in your rear-view. It could move along a lateral half-circle or a vertical one (or something in-between). If it went along a vertical half-circle it would end up upside-down and behind you; since it goes along a lateral one it also gets laterally-flipped, but because cars have left to right mirror symmetry , you don't notice (except for the words ambulance).
We have a huge bias evolutionary bias towards that type of lateral symmetry (e.g. in animals, other humans) and to things moving along a lateral plane while maintaining vertical orientation.