One from pub impedes jam, making traffic slower (5,4)

I believe the answer is:

speed bump

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'traffic slower' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

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(Other definitions for speed bump that I've seen before include "Sleepng policeman" , "Traffic-calming device" , "Traffic-calming measure (B, not H)" , "traffic checker" , "Sleeping policeman" .)

I've seen this clue in the Sydney Morning Herald.

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