California Code, Health and Safety Code - HSC § 41503.1

The state board may approve an attainment plan which achieves less emission reductions than 5 percent per year, or less than 15 percent every three years, as specified in Section 40914, if the state board determines that the district is unable to meet these requirements, despite the expeditious adoption of all feasible controls, or if the state board determines that the equivalent air quality improvement will be achieved through an alternate level of emissions reduction.