Charles Ellicott Commentary Judges 20:32

"And the children of Benjamin said, They are smitten down before us, as at the first. But the children of Israel said, Let us flee, and draw them away from the city unto the highways." — Judges 20:32 (ASV)

Said, Let us flee. In a later historical style, the plan of the feigned flight would have been mentioned earlier.

To the highways. This would have the double effect of allowing the ambush to cut off their retreat, and of dividing their forces at the point where the roads branched off.