GLOSSARY

Environmental Impact Report (EIR)

An environmental impact report (EIR) is the study California’s Environmental Quality Act (CEQA) requires when a public agency’s project may significantly affect the environment; it analyzes the impacts and alternatives and must be certified before the project is approved.

At a glance
Law
CEQA in California; federal projects use an EIS under NEPA
When it changes
A supplemental or subsequent EIR, or an addendum for minor changes
Key fact
The Foothill Gold Line reopened its EIR to cut planned station parking by 45%
THE EIR PATH UNDER CEQANotice ofPreparationScopingcommentsDraft EIRPubliccommentFinal EIRand responsesCertify andapprovePROJECT CHANGES LATER: SUPPLEMENTAL OR SUBSEQUENT EIRFoothill Gold Line: certified 2013, parking changes certified January 2021
The main steps of a CEQA environmental impact report. A significant change to a certified project generally reopens the process through a supplemental or subsequent EIR. Simplified.

For parking, the EIR matters in two ways. A parking structure can be part of the project the EIR analyzes, so its size and location are fixed in the certified project description. And when a project’s parking plan changes after certification, the agency usually has to reopen the analysis before it can build something different.

The main steps

  • Notice of Preparation and scoping, where agencies and the public say what the study should cover.
  • Draft EIR, with impacts, mitigation and alternatives, released for public comment.
  • Final EIR with responses to comments.
  • Certification by the lead agency, findings, and approval of the project.

What changed for parking analysis

Under SB 743, CEQA transportation analysis in California now measures vehicle miles traveled rather than intersection delay, which changes how parking supply and its effect on driving are evaluated. The companion report The Evolution of U.S. Parking Structures in 2026 found that traditional new parking builds face 12 to 24 months of environmental review, against roughly 6 to 9 months for projects framed as mobility hubs.

The Parkonomics view

Changing a certified project is not done with a staff memo. On the Foothill Gold Line, the Construction Authority issued a notice of preparation in June 2020, held a public hearing on the parking proposal, and certified a supplemental EIR in January 2021 that replaced four planned structures with surface lots. Plan right-sizing before the first certification when you can, because reopening it takes partnership, time and evidence.

Related terms: Right-Sizing, Park-and-Ride, Mobility Hub, Traffic Study, Transit-Oriented Development

Further reading: Designed to Park, or Designed to Impress?, The Evolution of U.S. Parking Structures in 2026