When the particular bonds issued for the projects to be constructed under this chapter and the interest thereon shall have been paid, or a sufficient amount shall have been provided for their payment and shall continue to be held for that purpose, and upon acceptance of the responsibility for operation and maintenance of the project by an agency of government legally authorized to do so, the authority shall cease to charge tolls or levy assessments for use of the project or projects, and thereafter the use of such projects shall be free.