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Explain xkcd: It's 'cause you're dumb.
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Bridge Clearance
A lot of the highway department's budget goes to adjusting the sign whenever the moon passes directly overhead.
Title text: A lot of the highway department's budget goes to adjusting the sign whenever the moon passes directly overhead.

Explanation

Bridges will often have signs indicating the height of the bridge, primarily to indicate to taller vehicles the potential hazard of driving under them. In this comic, there is a bridge with one of these signs, indicating a clearance of 10 feet and 6 inches, which is a normal clearance for a bridge to have. To its right, there's a sign which says "no bridge", indicating a clearance of 46 billion light years. This sign indicates a clearance approximately equal to the radius of the observable universe. This is because there isn't any bridge.

The title text references the fact that when the moon passes over the sign, the clearance would be reduced to the distance between Earth and the moon, as any vehicle driving there would need to be shorter, otherwise it would crash into the moon. Both of these signs ignore the fact that vehicles cannot be large enough that either of these clearances would be something necessary to account for while driving.

The title text says that the sign might crash into the moon during certain times. However, they also need to adjust it for any stars, which would be easier if their were just people living on the sign and adjusting it with some smart mechanism.

Transcript

[:There is a road with a fork and one side that goes to the left has a sign. The one that goes forward has a bridge and a clearance sign.]

[:left sign with bridge underlined, it has a horizontal line that is dotted with slashes.]

Bridge

Clearance

10 ft 6 in

[:right sign with No bridge underlined]

No Bridge

Clearance

46+ Billion light years



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