In reply to Roberttaylor:
I recall seeing some test data many years ago, before polystyrene helmets came along, when helmets were either "plastic" or composite (fibreglass or carbon fibre).
In terms of absorbing multiple impacts without transferring too much energy to the head/neck of the wearer composites were a clear winner. They also sat lower on the head and were less prone to bounce off the head (by excessive deformation) than plastic helmets. Plastics were cheaper and lighter however.
My first helmet was a fibreglass Phoenix Winter helmet which I retired when the rim became cracked which I thought must have weakened it. Years later (20 or so) I was having a clearout and came accross it. Before throwing it out I thought I'd test it to destruction. I dropped bricks on it several times from about 6 feet which barely scratched it so I took to whacking it it multiple times with end of a weightlifting barbell, it took 20 or so blows to break through the hard shell but the fibreglass matt was still holding it together. It then took multiple blows with a lump hammer to finally break it.
Totally unscientific and obviously I couldn't measure the impact force on the theoretical wearer's neck but pretty impressive nonetheless.
I somehow can't image a polystyrene helmet standing up to a fraction of that, but they a lot lighter!