Section 139: Where–

(a) any person is tried for the committal of any offence whereby any traffic sign is damaged or destroyed; or (b) any bridge is damaged for any of the reasons set out in section 138, a certificate under the hand of an engineer employed by a person responsible for the maintenance of such traffic sign or bridge, stating the amount of the cost of making good such damage or destruction, as the case may be, shall, without proof of signature, be prima facie evidence of such cost.