(2) In lieu of issuing bonds secured by unobligated net revenue under subsection (1) of this section, the treasurer, when authorized by the board, may execute and issue interest-bearing warrants drawn against funds created by and for operation and maintenance of the mass transit system in excess of current cash on hand, but not in an amount exceeding one-half of the estimated annual gross revenue for operation of the mass transit system for the next ensuing year. [1969 c.643 §29]