Expenditure of funds allocated by this act to the department of state highways or to a county road commission to pay the cost of drain assessments under Act No. 40 of the Public Acts of 1956, as amended, being section 280.1 to 280.623 of the Compiled Laws of 1948, imposed after the effective date of rules promulgated under this section, shall be made only if all apportionments of benefits of any nature or kind determined or made for the drainage of county roads or state highways are made in accordance with rules jointly promulgated by the department of state highways and the department of agriculture. The department of state highways and the department of agriculture jointly shall promulgate rules in accordance with and subject to Act No. 306 of the Public Acts of 1969, as amended, being section 24.201 to 24.315 of the Compiled Laws of 1948, to provide for an engineering determination of benefits based on the pro rata share of storm water runoff from the county roads or state highways within the drainage district in direct proportion to the total storm water runoff in the entire drainage district. Fifty percent of the cost of a drain assessment against the county for the drainage of county roads shall be paid by the county road commission from county road funds. Expenditures for such purposes shall be reported separately by each county road commission to the state highway commission. Section 16 applies to expenditures made pursuant to this section.