The helmet is for if you fall over and hit your head. You can die from a relatively minor blow to he head. It has nothing to do with being safe from cars. You could get hit by a car (more likely clipped and fall off) and bang your head, of course, but it isn't a car defense device.

Not being hit by cars is the best defense against being hit by cars. It is genuinely stupid to not wear some sort of reflector or light when riding in the dark. A driver can't avoid what he can't see. Volvo doesn't seem to have a monopoly on shiny surfaces.

I could see this being of more use if it were permanent. You could put it on your dedicated riding jacket and just always wear that jacket and no worry about the reflectors etc.