The California Gold Rush is the answer.
All modern Vehicles have<span> internal steel beams in the door, which helps to protect the passenger compartment.
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Just a few decades ago, passenger cabins were particularly vulnerable to a crash, especially if this occurred from the side of the car.
Car manufactures have spent considering time, research and money into developing light-weight and ergonomic-friendly safety measures to protect passengers.
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Because an infantry invasion of Japan, would have resulted in <em>Far</em> more casualties for both the americans and the japanese. Than the dropping of an atomic bomb.
