Backstreet regularly used to avoid congestion (3,3)

I believe the answer is:

rat run

(Other definitions for rat run that I've seen before include "Side street much used by cars in rush hours" , "Heavily used back street" , "Heavily-used minor road" , "Busy rush hour side road" , "Side-road used by vehicles to dodge traffic" .)

I've seen this clue in The Times.

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