(a) The security under this chapter shall be in such amount as the department may require, but in no case in excess of the limits specified in subsection (a) of Code Section 40-9-37. Every depositor of security shall designate in writing every person on whose behalf the deposit is made and may at any time change such designation, in writing, to include an additional person or persons; provided, however, that a single deposit of security shall be applicable only on behalf of persons required to furnish security because of the same accident.

(b) The department may, upon written notice to all parties involved, reduce or increase the amount of security ordered in any case within six months after the date of the accident if in its judgment the amount ordered is excessive or inadequate. In case the security originally ordered has been deposited, the excess deposit over the reduced amount ordered shall be returned to the depositor or his personal representative.