re: Why aren't double decker bridges more common?

I wish the entire BR area would build double decker interstates and make Siegen, Essen, Perkins, and Highland double deckers also. I’ve said for years that’s the answer to our traffic problems.

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As others have noted, soil composition must be such that the heavier loads can be supported. This factor alone decreases the number of available sites.

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Just widen the support of the bridge under water

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Helicopter medievacs can be handled by simply ambulancing the patients to a point where the airlift can be managed

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Not many regular bridge are able to be landed on by a helicopter, and anywhere that has a need for a double decker bridge would be close enough to a major metro area that an ambulance would probably be just as fast

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But another point that makes these bridges more difficult is that the trusswork must be stiffened so that vibrations on one level do not transmit to the other level.

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It’s been done before so why can’t it be done again?