(1) The board of county road commissioners may grade, drain, construct, gravel, shale, or macadamize a road under its control, make an improvement in the road, and may extend and enlarge an improvement. The board may construct bridges and culverts on the line of the road, and repair and maintain roads, bridges, and culverts.

  (2) The commissioners shall take over, construct, and maintain the bridges included in the proposed system of county roads provided in section 18 of this chapter, except bridges that are lawfully taken over for construction and maintenance in whole or in part by the state. When it is necessary to construct a bridge across a navigable stream over which a road under the control of the board of county road commissioners runs, the board may construct, repair, and maintain a bridge across the navigable stream with proper and adequate approaches to the bridge for the accommodation of traffic, and the bridge and the approaches to the bridge shall be of a width, size, strength, material, pattern, and design which the board considers suitable to meet the necessities and requirements of traffic and the relevant conditions, despite the fact that the bridge or the approaches to the bridge, or a portion of either, is within the limits of an incorporated municipality. The board of county road commissioners may borrow money, enter into contracts, and pledge the credit of the county in the manner and within the limitations provided by law for this purpose. For money borrowed, an obligation assumed, or a pledge of credit given, the bonds of the county may be issued and sold when authorized by the electors in the manner provided by law. The authority and powers granted in this section relative to bridges over navigable streams and the grant of that authority are retroactive and all acts of boards of county road commissioners, county boards of commissioners and other county officers, and all elections held, indebtedness incurred or authorized, and bonds approved or authorized by the electors of the county, to be issued and sold for the purpose and with the intent to provide for the construction, repair, and maintenance of bridges over navigable streams and their approaches are valid and binding acts and obligations. The obligations assumed, indebtedness incurred, pledge of credit given, and bonds issued or authorized by counties within this state for these purposes are valid.

  (3) The construction, improvement, and maintenance of roads, bridges, and culverts, shall be in accordance with plans and specifications furnished or approved by the county highway engineer, who shall have supervision of the construction. The county road commissioners have all the authority in respect to the roads, bridges, and culverts which is vested in highway officers in townships, including the right to condemn gravel for road purposes and to petition the county drain commissioner for an outlet drain as provided in section 8 of chapter 15.

  (4) The board of county road commissioners may maintain in its own name an action for injury to a county road, a part of the road as laid out and established, or to an improvement on the road. Money recovered in the action shall be paid to the county treasurer and shall be credited to the county road fund.

  (5) When the board of county road commissioners decides to build or rebuild a road or bridge by a method other than by employment of day labor and purchase of necessary materials, the board shall advertise for sealed bids for the work, if the work involves an expenditure of an amount greater than $20,000.00. The board may reject any and all bids and may do the work by day labor, purchasing the necessary materials and employing the necessary labor. If the board decides to do the work by day labor, the plans and specifications together with all bids received on the plans and specifications, and the reason, in writing, for not letting the job by contract, shall be filed in the office of the county clerk. If a bridge is to be repaired or built at a cost greater than $500.00, the county road commissioners may apply to the state transportation department, which shall assign a competent engineer to review and report on plans that may have been prepared by the county highway engineer, or decide regarding the merits of each plan on which tenders may have been received before the contract was let, and pass on the completed structure before the contractor for that contract is paid.