California Code, Public Utilities Code - PUC § 131100

(a) The Legislature, by the enactment of this chapter intends a county transportation authority or the commission, pursuant to a county transportation expenditure plan adopted pursuant to Section 131055, to use any additional funds provided by this chapter to supplement existing local revenues being used for public transportation purposes listed in the plan. The Legislature further intends that the funds provided pursuant to this chapter shall not replace funds previously provided by property tax revenues for public transportation purposes. The nine-county San Francisco Bay area is further encouraged to maintain its existing commitment of local funds for public transportation purposes.

(b) Any tax revenue generated pursuant to this chapter shall be expended in the county of origin, except that tax revenue generated may be expended within and outside the county of origin if so provided in the adopted county transportation expenditure plan. However, the tax revenues may be exchanged for federal or state funds available to another county or local government for transportation purposes if the exchange will benefit the county of origin.