Well, when I was hit by a car and hitting the asphalt head first as a teenager, I found it adequate, that the helmet was cracked to pieces afterward. Did it save my life? I don't know, but with the helmet I only had a light concussion (and broken leg) compared to very possible skull crack.

Compressed polystyrene I know only from light accidents, but it has been a while and I suppose todays helmets are a bit more durable. (But luckily never had to find out, if they fare better nowdays. Also I learned to fall and only rarely wear a helmet nowdays)