[mung]

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.

[tonyedgecombe]

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.

[walshemj]

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

[tonyedgecombe]

Ugh ...

[BillTheBiker]

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.