The state transportation department and department of state police shall adopt a manual and specifications for a uniform system of traffic control devices consistent with the provisions of this chapter for use upon highways within this state. The manual shall correlate with and so far as possible conform to the federal manual then current as approved by the United States Department of Transportation, Federal Highway Administration, and may be revised whenever necessary to carry out the provisions of this act. It is the policy of this state to achieve, insofar as is practicable, uniformity in the design, shape, and color scheme of traffic signs, signals, and guide posts erected and maintained upon the streets and highways within this state with other states. Definitions and meanings found in the manual adopted under this section are supplemental to the definitions in chapter I. However, if a definition or meaning found in the manual adopted under this section conflicts with a definition in chapter I, the definition in chapter I prevails.