There is no gold standard for assessing when or whether a project’s impacts are environmentally significant. (Protect the Historic Amador Waterways v. Amador Water Agency(2004) 116 Cal.App.4th 1099, 1107 (Amador Waterways).) When considering the significance of an environmental impact, agencies employ thresholds of significance, which establish the level at which the impact is found to be significant. (Communities for a Better Environment (2002) 103 Cal.App.4th 98, 110 (Communities for a Better Environment), overruled on other grounds by Berkeley Hillside Preservation v. City of Berkeley (2015) 60 Cal.4th 1086.) Thresholds of significance may be quantitative standards, qualitative standards, or sets of criteria by which the significance of a given environmental effect may be determined. (CEQA Guidelines, § 15064.7(a).) Impacts found to be beneath an applicable threshold will “normally” be found insignificant, and impacts found to be above an applicable threshold will “normally” be found significant. (Ibid.) Thresholds of significance must be based, to the extent possible, on scientific and factual data supported by substantial evidence. (See CEQA Guidelines, §§ 15064(b)(1), 15064.7(b)-(c).)
Adopting Thresholds of Significance
Agencies are encouraged to adopt thresholds of significance for general use to promote consistency, efficiency, and predictability. (See CEQA Guidelines, §15064.7(b); Pub. Resources Code, § 21082; Communities for a Better Environment, supra, 103 Cal.App.4th at p. 111.) When creating a threshold for general use, an agency must meet certain procedural requirements. (CEQA Guidelines, § 15064.7(b).) General use thresholds must be (1) adopted by ordinance, resolution, rule, or regulation, (2) developed through a public review process, and (3) supported by substantial evidence. (Ibid.) A general use threshold adopted without complying with these procedural requirements is invalid. (See Golden Door Properties, LLC v. County of San Diego (2018) 27 Cal.App.5th 892, 902-903 [a county guidance document containing a recommended efficiency metric for assessing the significance of a project’s greenhouse gas emissions constituted a threshold of significance for general use and was invalided because it did not adhere to section 15064.7(b)’s procedures].)
While agencies are encouraged to adopt and publish thresholds of significance for general use, they are not required to do so, and may adopt thresholds for discrete projects without complying with the procedural requirements of general use thresholds. (See CEQA Guidelines, § 15064.7(b), 15064(b)(2); Oakland Heritage Alliance v. City of Oakland (2011) 195 Cal.App.4th 884, 896 (Oakland Heritage) [CEQA Guidelines section 15064.7 “does not require a public agency to adopt [general use] significance thresholds [and] it does not forbid an agency to rely on standards developed for a particular project.” (original emphasis)].)
When establishing a threshold, agencies may use standards developed by experts or accept the opinion of experts regarding the appropriateness of a significance standard. (See Mount Shasta Bioregional Ecology Ctr. v. County of Siskiyou (2012) 210 Cal.App.4th 184, 205, 208-209 [upholding the use of noise significance standards based on expert reports]; Napa Citizens for Honest Gov’t v. Napa County Bd. of Supervisors (2001) 91 Cal.App.4th 342, 362-363 [upholding the use of a traffic impact significance standard developed by EIR drafters].) Developing an ironclad definition of significance is not always possible, given that the significance of an activity varies with the setting. (CEQA Guidelines, § 15064(b)(1).) Accordingly, an agency should consider the nature of the area affected by a project when adopting a threshold. (See ibid.; Berkeley Keep Jets Over the Bay Com. v. Board of Port Cmrs., 91 Cal.App.4th 1344, 1380-1383; Lotus v. Department of Transp. (2014) 223 Cal.App.4th 645, 654-658 & fn. 7; Clover Valley Foundation v. City of Rocklin (2011) 197 Cal.App.4th 200, 243-244.)
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