In Silicon Valley Taxpayers' Assn., Inc. v. Santa Clara County Open Space Authority (2008) 44 Cal.4th 431, the California Supreme Court held that the passage of Proposition 218 undermined the legislative-function rationale for the deferential standard of review on the substantive requirements for assessments. (Silicon Valley, supra, 44 Cal.4th at pp. 450-451.) "Before Proposition 218 became law, special assessment laws were generally statutory, and the constitutional separation of powers doctrine served as a foundation for a more deferential standard of review by the courts. But after Proposition 218 passed, an assessment's validity, including the substantive requirements, is now a constitutional question. 'There is a clear limitation ... upon the power of the Legislature to regulate the exercise of a constitutional right." (Silicon Valley, supra, 44 Cal.4th at p. 448.) The court concluded that an agency's determination whether an assessment met the substantive requirements of article XIII D, section 4 is subject to independent rather than deferential judicial review. (44 Cal.4th at p. 450.)