Cal. Pub. Util. Code § 142257

(a) The expenditure plan shall specify the amount and the formula by which the retail transactions and use tax shall be allocated to each city and the county for local transportation purposes determined to be priority projects by local governments to which funds are allocated.
For purposes of this subdivision, the population of the county is the population of the unincorporated area of the county.
(b) Prior to the authority allocating funds, each local government shall certify to the authority that the funds will not be substituted for property tax funds which are currently utilized to fund existing local transportation programs. If the local government is unable to segregate property tax revenues from other general fund revenues which cannot be so distinguished, substitution of funds from the authority for general funds is also prohibited.
(c) The authority shall require that local governments to which funds are allocated to separately account for those funds and maintain records of expenditures in accordance with administrative code requirements adopted by the authority.