It is not very clear in the book however we had the same problem and after scowering the rules can only come to the conclusion that they work like any other unit except that if they fail to refrain from pursuit and were to pursue into a friendly unit / terrain then they either inflict or take impact hits as normal.
Also in the rule book, if chariots overrun in friendly unit or terrain, the chariot stops before hitting them. This doesnt apply to a fleeing chariot, who causes impact to any unit he's fleeing through and if he's fleeing through woods/difficult terrain he get 1d6 Str6 hits.
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