When the LORD routed Sisera's army before Barak, how did Sisera escape?

Answer: He fled on foot

Sisera abandoned his chariot and fled on foot to the tent of Jael, the wife of Heber the Kenite.

“And in front of him the LORD routed with the sword Sisera, all his charioteers, and all his army. Sisera abandoned his chariot and fled on foot.”— Judges 4:15 (BSB)