(a) Every person licensed as an operator shall write the person's usual signature, in a manner acceptable to the Department, in the spaces provided for that purpose on the license certificate and such license shall not be valid until the certificated is so signed.
(b) The licensee shall have such license in the licensee's immediate possession at all times when driving a motor vehicle and shall display the same upon demand of a uniformed police officer, justice of the peace, peace officer or motor vehicle inspector. It shall be a defense to any charge under this subsection if the person so charged produces in court an operator's license theretofore issued to such person and valid at the time of the arrest.