Something I've noticed on my newer car that my older, similar model didn't have, is that the empty spaces in the body structure have been filled in with heavy-duty styrofoam. It's like the styrofoam in a motorcycle helmet-very stiff and hard. Presumably it's there to absorb additional energy by compressing as the structure collapses.
As far as retrofitting, it seems like it would be almost impossible to predict exactly what effect it would have, although you could probably guess that it would increase the rate at which energy was absorbed (more energy absorbed per unit of body deformation), and on a car whose safety performance might be improved by this, it might be helpful. An awful lot of guessing and conditional language in that statement, though.
Small unibody vehicles seem like they may benefit from this treatment. It seems like it would allow the structure to absorb more energy in the limited amount of distance available to compress. It would increase the G-loading during the compression, though, increasing the forces on the occupants.
Probably not helpful for body-on-frame units, where the problem may be that the structure is already too stiff.
I haven't seen expanding spray-foam that looks like styrofoam when it hardens, but I suppose it might be available.
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