California Code, Revenue and Taxation Code - RTC § 7295

(a) Notwithstanding any other law, the County of San Mateo may, in accordance with the requirements of the Bay Area County Traffic and Transportation Funding Act (Division 12.5 (commencing with Section 131000) of the Public Utilities Code) relating to the imposition of transactions and use taxes, impose a transactions and use tax for the support of countywide transportation programs at a rate of no more than 0.5 percent that would, in combination with all taxes imposed in accordance with Part 1.6 (commencing with Section 7251), exceed the limit established in Section 7251.1.

(b) The county shall not act pursuant to this section if a retail transactions and use tax has been imposed pursuant to subdivision (b) of Section 103350 of the Public Utilities Code.