In a published opinion filed March 27, 2025, the Fourth District Court of Appeal (Div. 1) reversed the trial court’s judgment denying a writ petition, and held that two screening thresholds of significance for vehicle miles traveled (VMT) impacts adopted by the County of San Diego as part of its 2022 Transportation Study Guide were invalid because they were unsupported by any substantial evidence.  Cleveland National Forest Foundation, et al. v. County of San Diego (2025) ___ Cal.App.5th ___.

CEQA Thresholds of Significance Generally And With Respect to VMT

The CEQA Guidelines encourage public agencies to develop and publish thresholds of significance to promote consistency in their significance determinations.  (CEQA Guidelines, § 15064.7(b), (d).)  Such thresholds are used to predict when a certain type of impact will normally be insignificant; a threshold of significance is thus defined as “an identifiable quantitative, qualitative or performance level of a particular environmental effect, non-compliance with which means the effect will normally be determined to be significant by the agency and compliance with which means the effect normally will be determined to be less than significant.”  (§ 15064.7(a).)

OPR’s proposed revisions to the CEQA Guidelines providing that VMT “[g]enerally … is the most appropriate measure of transportation impacts” – which resulted from SB 743’s legislative direction in 2013 (see, Pub. Resources Code, § 21099(a)(7), (b)(1), (c)(1)) – were adopted in December 2018 and took effect on July 1, 2020.  The basic aims of this CEQA “sea change” were to achieve denser infill development, reduce reliance on individual vehicles, and improve mass transit, all to the end of reducing climate-changing GHG emissions.  The relevant Guideline defined VMT as “the amount and distance of automobile travel attributable to a project” and provided “VMT exceeding an applicable threshold of significance may indicate a significant impact.”  (Guidelines, § 15064.3(a), (b)(1).)  Conversely, projects located within one-half mile of an existing major transit stop or high quality transit corridor stop, or that decrease project area VMT compared to existing conditions, should be presumed to have a less-than-significant transportation impact.  (Id., subd. (b)(1).)

About the same time these Guidelines revisions were adopted, OPR published its “Technical Advisory on Evaluating Transportation Impacts in CEQA” (“Technical Advisory”) as “a resource for the public to use at their discretion” and which made “recommendations regarding assessment of VMT, thresholds of significance, and mitigation measures.”  Among other things, the Technical Advisory “recommends using quantitative VMT thresholds” linked to GHG reduction targets “when methods exist to do so” and suggests “a per capita or per employee VMT that is fifteen percent below that of existing development may be a reasonable threshold” of significance for a specific project’s transportation impacts.  (Technical Advisory, at pp. 8, 10.)

The Technical Advisory does not recommend applying the “15 percent standard” as a threshold of significance for every project, however, and recognizes that other appropriate thresholds may be used to “screen out VMT impacts” for some projects by determining they should be expected to have “a less-than-significant VMT impact without conducting a detailed study.”  (Id., at p. 12.)  OPR thus suggested four screening thresholds for:  (1) “small projects … that generate or attract fewer than 110 trips per day”; (2) projects in “low-VMT” areas already below the 15 percent standard; (3) projects within one-half mile of either “a major transit stop” or a “stop along a high quality transit corridor”; and (4) 100 percent affordable housing projects in infill locations.  (Id., pp. 12-15.)  For projects not screened out, OPR recommends agencies apply its 15 percent standard of per capita VMT for residential projects or per employee VMT for office projects.  (Id. at pp. 15-16.)

County’s Transportation Study Guide And Its Challenged VMT Thresholds

In September 2022, County adopted by resolution its Transportation Study Guide to implement SB 743’s directives.  It included screening thresholds that could obviate detailed, project-specific VMT analysis, but provided that where no screening threshold applies a detailed VMT evaluation is required and that OPR’s 15 percent threshold of significance applies.

At issue in this case were two of County’s screening thresholds:  its “infill” and “small project” thresholds.  (Five other thresholds in County’s Transportation Study Guide were not challenged in the litigation.)  The “infill” threshold is for “projects located in infill village areas” in “the unincorporated County likely to be provided with transit in the future.”  For purposes of this screening threshold, County’s infill consultant identified infill areas using baselines of housing and intersection density and job accessibility associated with urban areas, and where an infill area’s boundary was not coextensive with that of a village in which the infill area was located, “County expanded the infill area’s boundary to match that of the village.”  No VMT analysis was used to identify the locations of these “infill village areas.”

The “small project threshold exempts from VMT analysis a residential or office project that is expected to generate fewer than 110 [daily] automobile trips.”  Here, County simply adopted OPR’s recommended threshold without adjustment for local VMT or how it compares to the statewide average.

The Court of Appeal noted the appellate record contained only “a smattering of information concerning VMT in the County,” which indicated that “the infill locations and associated villages generally had per capita VMT values higher than the County average (and much higher than OPR’s 15 percent standard).”

Petitioners (later Appellants) filed a writ petition claiming that the infill threshold was invalid because it is qualitative (as opposed to a quantitative standard based on available VMT data), and that both thresholds lacked supporting substantial evidence that projects “screened-out” thereby would generally cause a less-than-significant VMT impact because County’s justifications and assumptions in that regard were not shown to be valid for local conditions.  The County defended its screening thresholds, arguing that the infill threshold was properly based on both quantitative data (household and intersection density and job availability) and the “generally accepted assumption” that infill development does not significantly impact VMT.  It argued the small project threshold should be upheld essentially because it is identical to a threshold recommended in OPR’s Technical Advisory.

The trial court accepted County’s arguments and denied the petition, but upon Petitioners’ appeal, the Court of Appeal reversed.

The Court of Appeal’s Opinion

Standard of Review

Reviewing the County’s adoption of thresholds of significance as “quasi-legislative action” under CEQA’s familiar “prejudicial abuse of discretion” standard, the Court of Appeal noted that such an abuse “is established if the agency has not proceeded in a manner required by law or if the determination or decision is not supported by substantial evidence.  ([Pub. Resources Code,] § 21168.5.)”  The courts independently review claims under the “failure-to-proceed” prong while according “deference to [an agency’s] factual conclusions, as long as they are supported by substantial evidence.”  (Citing Golden Door Properties, LLC v. County of San Diego (2018) 27 Cal.App.5th 892, 901-902.)  Absent “a threshold mandated by statute,” agencies like the County have “substantial discretion in determining the appropriate threshold of significance to evaluate the severity of a particular impact.”  (Quoting Mission Bay Alliance v. Office of Community Investment & Infrastructure (2016) 6 Cal.App.5th 160, 192.)  The CEQA Guidelines are afforded great weight in interpreting CEQA unless “clearly unauthorized or erroneous,” and OPR’s Technical Advisory is relevant to interpreting its Guidelines as it reflects OPR’s “expertise and technical knowledge… pertain[ing] to a complex technical statute[.]”  (Citing California Building Industry Assn. v. Bay Area Air Quality Management Dist. (2015) 62 Cal.4th 369, 381, 389 -390.)

County’s “Infill” Threshold Held Invalid Because Its Assumptions Lack Substantial Evidence Support But Not Because It Is Qualitative In Nature

The Court of Appeal quickly dispensed with appellants’ legal challenge to the infill threshold, which was based on the argument that it must be quantitative, under Guidelines section 15064.3(b)(3), unless existing models or methods are unavailable to develop such a standard.  This argument failed because the cited Guidelines section “relates to specific projects and not thresholds of significance” and “addresses the circumstance when quantitative data is unavailable to estimate the VMT ‘for the particular project being considered.’”  (Citing Guidelines, § 15064.3(b)(3).)  Further, OPR’s Technical Advisory itself recommends screening thresholds based on qualitative project characteristics (e.g., size, transit availability, affordable housing); thus, “as a conceptual matter, CEQA does not prohibit the County from adopting a qualitative infill threshold.”

However, that wasn’t the end of the story – the Court next held the infill screening threshold was invalid because its underlying assumptions are not supported by substantial evidence showing development consistent with it will generally have a less-than-significant impact on VMT under local conditions.  The purpose of a threshold of significance is to identify when a particular environmental effect would normally be deemed insignificant, and the threshold must be supported by substantial evidence.  (CEQA Guidelines, § 15064.7(a), (b).)  “Substantial evidence” is “enough relevant information and reasonable inferences [therefrom] that a fair argument can be made to support a conclusion, even though other conclusions might also be reached.”  (§ 15384(a).)  It “include[s] facts, reasonable assumptions predicated upon facts, and expert opinion supported by facts” but not “[a]rgument, speculation, unsubstantiated opinion or narrative, [or] evidence which is clearly inaccurate or erroneous[.]”  (Id., subds. (a), (b).)  Per the Court, while it must resolve evidentiary conflicts and indulge all reasonable inferences in support of the agency’s findings in conducting substantial evidence review: “Ultimately, substantial evidence must have a firm factual foundation.”  The Court framed the issue regarding the infill threshold as follows:

In the context of this case, the County was required to make some showing that development consistent with the adopted infill threshold will normally likely result in an insignificant transportation effect.  In other words, will development in infill and village areas, as defined by the County, generally result in per capita VMT that is insignificant, even if it does not always do so?”

(Slip Opn. at p. 14, citing Guidelines, § 15064.7(a), footnote omitted.)

The Court held that no substantial evidence supported the required showing.  County’s justification for the infill exemption and its decision to expand infill area boundaries throughout contiguous villages was the “general assumption that development in more dense areas, including infill development, does not significantly impact VMT.”  But the assumption was overly broad and also undermined by reports by County’s own infill consultant contained in its Transportation Guide’s appendix, which “stated that defining appropriate screening criteria ‘would require evidence to support the determination that projects in these locations would have a less than significant transportation impact and meet the intent of [SB] 743.’”  The consultant also pointed out what the Court had earlier observed, i.e., that the record evidence showed that most County locations – presumably including “infill” ones – generated VMT at or around, not below, the regional mean, which does not support the assumption underlying the screening threshold.  Nor did the CAPCOA Handbook or County staff’s opinions based thereon provide the requisite substantial evidence support because the Transportation Guide did not rely on the Handbook’s VMT-related methodologies and the Handbook did not define or describe infill or density in a way analogous to County’s methodology.  Per the Court:

It is not enough to say that infill development is better than non-infill development in terms of transportation impact or that increasing development density generally reduces VMT.  The question is not a relative one, but rather one of significance versus insignificance as to the specific infill and village areas the County has identified where projects can be developed without the need for studying VMT impacts.”

The bottom line was that the “evidence” County relied on reflected only “unsubstantiated opinions about infill development generally” and was not based on “facts showing how often development in its designated infill and village areas will not cause a significant transportation-related impact as measured by VMT.”  (Emph. added.)  The Court opined that the case law confirmed its “common sense interpretation of the Guidelines’ requirements for significance thresholds” by explaining how use of statewide data must be justified by adequate explanation or adjustment to fit local circumstances.  (Citing Golden Door, 27 Cal.App.5th at 898, 904-905, and CEQA Guidelines, § 15064.7(c) [adoption of another agency’s threshold must be supported by substantial evidence].)

County’s “Small Project” Threshold Also Lacked Substantial Evidence Support

The Court also invalidated County’s small project screening threshold – projects generating less than 110 daily vehicle trips – as lacking the required substantial evidence support showing that projects screened out under it would likely cause a less-than-significant transportation effect in the County.  The Court rejected County’s arguments that it was allowed to simply adopt another agency’s significance threshold without such a showing and that statewide goals can automatically be used as local thresholds.  Rather, “substantial evidence in this context includes evidence that the threshold applies as intended in the local conditions.”  (Citing Center for Biological Diversity v. Department of Fish & Wildlife (2015) 62 Cal.4th 204, 226-227 [as explaining “statewide criterion is an acceptable significance threshold only if there is substantial evidence to support its application to a specific project”].)  OPR’s recommendation alone could not excuse County’s failure to provide any evidentiary support for applying the no significant impact assumption under local conditions.  The County acknowledged OPR’s small project threshold was based on evaluating projects statewide, not in a single jurisdiction, and it made “no effort… to develop any evidence that small projects generating 110 or fewer trips are likely to cause a less than significant transportation effect in San Diego County.”  The Court opined:  “This burden is not an onerous one, but it must be addressed.”

Conclusion and Implications

While agencies have substantial discretion in developing and determining thresholds of significance – including “screening thresholds” of the type at issue in this case – to evaluate the significance of projects’ environmental impacts, such thresholds must be supported by substantial evidence and (as emphatically demonstrated by this case) judicial review for substantial evidence is not toothless.  As our Supreme Court taught nearly a decade ago in an analogous context – involving analysis of the significance of an individual development project’s GHG emissions – an agency cannot simply assume that what is true of the whole state is equally true of its local constituent jurisdictions.  (See my December 2, 2015 post on the Center for Biological Diversity case here.)  Accordingly, agencies intending to rely on generalized assumptions, or to adopt another agency’s significance threshold, must be careful to “connect the dots” with substantial evidence showing the assumptions or adopted threshold remain valid and applicable under the conditions obtaining in the agency’s local jurisdiction.





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