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Actually, it is simple physics. Given the forces of friction on the car in an accident the lighter car will be most likely to break free of the forces of friction and be pushed infront of the car that hit it. This would increase the time of the moment of impact thus decreasing the severity of the impact. Couple this with the improvements in airbags and other impact restraints such as pretensioning belts and you're probably much better off in the newer car. Now, if the lighter car is to be "sandwiched" between the oncoming car and something else, your advantage might be to be in the steel tank.
That's preposterous and you've obviously not seen many frontal crashes of a larger car/truck with lots of steel and a newer lighter car with lighter steels and composite panels. Also, if your theory was valid, the lighter cars should bounce off of semi-trucks. Have you ever seen an accident with a semi-truck and a passenger car? And for the record your "pretensioning belts" theory doesn't apply, because in my "antique" there is an explosive charge that sets the passenger seat belts upon impact. The lighter car isn't going to bounce off, but absorb a horrendous amount of energy that the heavier car/truck imposes upon it by its mass. Now if we're talking about a brand new wagon similar to mine, of course the newer one should be a bit safer (side impact bags), but that doesn't mean mine is not safe or an "antique".