In case any tolls or charges remain unpaid for a period of 30 days after they become payable, they shall become delinquent and a penalty of 10 percent shall be added thereto, and they bear interest at the rate of 1 percent per month or fraction thereof.
Source:
Section 35520.40, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=WAT§ionNum=35520.40..