It shall be the duty of the State Aid Engineer to advise with the boards of supervisors of the several counties on all matters of policy, use of funds, priority of construction, uniform standards for state aid roads, safeguards in accounting methods, and other related matters and to cooperate with the several boards of supervisors on all matters connected with the laying out and construction of the state aid system of county roads. The State Aid Engineer shall promulgate, as soon as practicable, such uniform and reasonable rules and regulations as he may deem necessary to effectuate a proper designation of state aid roads to be constructed in each county, the methods for determining priority of construction, the making of surveys, and the preparation of plans and specifications for the construction of state aid roads, and to provide a uniform system of accounting in the expenditure of state aid road funds. The State Aid Engineer, after conferring with the Chief Engineer of the State Highway Department, shall prepare and promulgate uniform design standards and specifications for the construction of the state aid road system, which said uniform design standards and specifications may be modified or amended from time to time as the State Aid Engineer may deem necessary. Such standards may be in one or more classifications, according to types and kinds of roads. After such uniform design standards and specifications have been prepared and approved by the State Aid Engineer, the boards of supervisors shall apply the same to all new construction of state aid roads in their counties and, unless not practicable and feasible, to reconstruction of old roads on the state aid road system; but no deviation from such uniform standards and specifications shall be made without the approval of the State Aid Engineer.

It shall also be the duty of the State Aid Engineer to advise and cooperate with the boards of supervisors in the selection and designation of the county roads which are to be made a part of the state aid road system, as provided in this chapter, and to approve or disapprove the selection of roads to be made a part of the state aid road system by the boards of supervisors.

The State Aid Engineer shall finally approve or disapprove all contracts advertised and let by any board of supervisors for the construction or reconstruction of state aid roads and he shall approve or disapprove any or all force account estimates for such construction. If disapproved, he shall give a notice to said county of his disapproval and state each reason, and he shall give the said county time to cure the defects, or such parts thereof as may be necessary to cure, so that the county may receive its share of state aid.

All proposals covering work to be performed by any county with its own forces on state aid roads and all force account estimates submitted for approval shall be on forms prepared for the purpose by the State Aid Engineer. Such forms shall be in such detail and based upon such cost accounting rules and regulations as may be prescribed from time to time by the State Aid Engineer, but in no event shall the purchase of any road machinery or other general equipment out of the state aid road funds be allowed or permitted by such rules and regulations. Force account estimates may include a reasonable rental for machinery or equipment, and the reasonableness of the rental so estimated and as actually paid shall be subject at all times to modification, revision, approval, or disapproval of the State Aid Engineer and under the cost accounting rules and regulations promulgated by him.

The State Aid Engineer and such assistants as he may designate shall supervise and inspect all state aid road projects as the work progresses. Upon final completion of any such project, the State Aid Engineer shall cause a final inspection to be made of such project for the purpose of determining whether such project has been completed satisfactorily in accordance with the plans and specifications; and if satisfactorily completed, the State Aid Engineer shall approve payment of the final estimate on such project. No progress or final estimate, either on a contract or a force account project, shall be paid unless approved in such manner by the State Aid Engineer, and on all such contracts or force account projects a percentage of not less than two and one-half percent (2½%) nor more than ten percent (10%) of each estimate thereon paid shall be retained until final acceptance of such project; provided, however, the amount retained by the prime contractor from each payment due the subcontractor shall not exceed the percentage withheld from the prime contractor.