Approval by county planning board required. All applications for site plan approval for signs on a county road shall be submitted to the county planning board for its review and recommendations and, where applicable, approval where required by state statute or county requirements. The applicant shall furnish proof of such submission within 10 days of the submission of his application to the township reviewing board by presenting a copy of his site plan with an indication from the county that it has been filed with it. Any application for site plan approval shall not be deemed complete in the absence of proof that it has been filed with the county planning board, if required. If the county planning board has failed to grant or deny approval of the site plan at the time of approval of the applicant's application, such approval shall be conditioned on approval of such site plan by the county planning board.