"Weighted average useful life" of a project or projects means:
For purposes of this computation, the useful life of land is fifty (50) years. The useful life of all other assets comprising the project shall be conclusively evidenced by a certificate of the department based on its experience in maintaining transportation systems. The weighted average useful life of any project shall be determined as of the later of the date on which the project is expected to be placed in service and the date on which the bonds or notes are issued.
IC 8-14.5-2-14