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To decrease or lessen. That group encourages the government to build down on wasteful spending. Give the traffic some time to build down before you leave the office—otherwise, you'll just sit on the highway for an hour.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
[for traffic] to reduce in volume or diminish. At about six, the going-home traffic begins to build down. When traffic builds down, I leave for home.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
Reduce, diminish, as in Owing to increased vigilance, traffic in narcotics is finally building down. This term, the antonym of build up, came into use about 1980 with regard to reducing the stockpile of nuclear weapons and soon was applied more widely.
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.