The county commissions of the several counties of this state have general superintendence of the public roads, bridges, and ferries within their respective counties so as to render travel over the same as safe and convenient as practicable. To this end, they have legislative and executive powers, except as limited in this chapter. They may establish, promulgate, and enforce rules and regulations, make and enter into such contracts as may be necessary or as may be deemed necessary or advisable by such commissions to build, construct, make, improve and maintain a good system of public roads, bridges, and ferries in their respective counties, and regulate the use thereof; but no contract for the construction or repair of any public roads, bridge, or bridges shall be made where the payment of the contract price for such work shall extend over a period of more than 20 years.