1
general::
Phrase(s): build down
[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.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms