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Why do bridge engineers use probable traffic loads for very long bridges?

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bridge The multiple-span Seto Great Bridge over the Inland Sea, linking Kojima, Honshu, with Sakaide, Shikoku, Japan.

Bridge engineers use probable traffic loads for very long bridges because designing for the maximum conceivable load is economically unreasonable.1

On short spans (less than 30 meters), it is possible for the maximum load to occur, such as when multiple heavy trucks cross simultaneously in both directions. However, on spans of a thousand meters or more, the probability of the bridge being filled bumper-to-bumper with heavy traffic in every lane at the same time is so remote that it does not justify the immense cost of such a design.1

For example, when designing the George Washington Bridge, engineer Othmar Ammann calculated a maximum conceivable load of 69,000 kg per meter but designed the structure to carry only 12,000 kg per meter, demonstrating that longer bridges do not require the same load-bearing capacity as shorter ones.2