A planning commission shall submit two copies of the proposed subdivision plat to the department if such proposed subdivision includes or abuts on any part of the state highway system. The department, within 30 days of receipt of the plat, shall recommend approval and note its recommendation on the copy to be returned to the planning commission or recommend rejection. Failure of the department to act within this 30 day period shall constitute approval. If the plat is recommended for rejection, the reasons for rejection and requirements for approval shall be given the commission in writing; but such recommendation shall be advisory only and shall not be binding on the planning commission concerned.