The separate and uncoordinated development of public highways and urban public transportation systems is wasteful of this state's natural and financial resources. It is the public policy of this state to encourage wherever feasible the joint planning, construction and maintenance of public highways and urban public transportation systems serving common geographical areas as joint use facilities. To this end the legislature declares it to be a highway purpose to use motor vehicle funds, city and town street funds or county road funds to pay the full proportionate highway, street or road share of the costs of design, right-of-way acquisition, construction and maintenance of any highway, street or road to be used jointly with an urban public transportation system.