A collision involving multiple vehicles, or a general accumulation of things that have built up over time.
From 'pile' (Old French 'pile') plus 'up' (Old English 'upp'). The term became common in the 20th century with automobiles.
Highway pileups on foggy days happen because people follow each other's taillights blindly—physics research shows drivers need 10-second gaps in poor visibility, yet many use 1-second gaps.
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