Mo. Rev. Stat. § 227.110

The state highway system shall be constructed in accordance with plans, specifications, estimates of cost, and contracts entered into with the commission, in the name of the state of Missouri. Said roads shall have a right-of-way not less than forty feet wide, or as much wider as may be necessary. Bridges and culverts over all nonnavigable streams which are located at points where such streams intersect the state highway system shall be regarded as part of the state highway.