Sure, but the question remains whether mandatory helmets would prevent a significant number of car injuries and deaths (I'd say that's quite plausible). Whether cars have other safety features already is neither here nor there.

A helmet in a car would limit visibility and movement and is completely impractical, not to mention mildly pointless.

On a bike you are exposed. In a car, you are not. On a bike you travel around 30km average. In a car, up to 100km average.

I think you will find the average cycling speed is a lot slower than that, especially around urban areas where most of the risks are.

See the cyclists in London or any big city blasting though a red light and you would change your mind

Ugh ...

Helmets go on top of your head, they shouldn't limit visibility. The practicality, and point, would be protecting you in the event of a collision.

When travelling at 100km with no seatbelt (legal in many areas), in a collision, you become exposed rather suddenly.