Select a county to see whether accessory dwelling units (ADUs) can mitigate a single-family
development's VMT impact. Explore the mitigating effect of market-rate ADUs, deed-restricted ADUs (low-income
or senior housing), and mixed ADU inventories.
County Map
Fall/Spring 2025 data
ADU-Inclusive Development is Fully Self-Mitigating ADU-Inclusive Development is Partially Mitigating — Market-Rate ADUs Help ADU-Inclusive Development is Partially Mitigating — Restricted ADUs Only
County Detail
Select a county on the map, or from the dropdown, to see its VMT baseline, threshold, and ADU mitigation results.
▸ Methodology & Data Sources
VMT & threshold
VMT per capita = home-based trips by personal motorized vehicle (private auto, carpool, on-demand auto) ÷ resident population, computed from Replica Places synthetic population and trip data (Fall/Spring 2025). Group-quarters residents (institutional populations) are excluded from both the numerator and denominator.
Threshold = 85% of existing county VMT per capita — the standard OPR / SB 743 significance threshold (15% below baseline).
Housing category profiles
Single-family (market-rate): local household trip rate × local average trip distance ÷ local occupancy, all observed directly in Replica.
ADU categories (senior-restricted, income-restricted, market-rate unrestricted; a 4th owner-used/inert category contributes zero added population and zero added trips):
Distance: Replica trips by renters in single-family-zoned buildings — the closest available signal for "occupies a secondary unit on a single-family lot," since Replica has no explicit ADU flag. Senior category is further restricted to retirees (not in labor force); income category uses the county-specific CA HCD Low Income limit for the household's size.
Rate: ITE Trip Generation Manual (12th ed.) rate for the matching land use — 210 (Single-Family), 251 (Senior Adult Housing), 223 (Affordable Housing), or 220 (Multifamily Low-Rise) — normalized by the ratio of the local Replica single-family rate to ITE's own single-family rate.
Occupancy: Census ACS 2019–2023 5-Year PUMS, studio/1BR renter households, split by income tier.
How the map tiers are defined
Self-mitigation achievable: a blended project at 100% ADU-inclusion (every SFH unit paired with one ADU of the county's single best-performing category, fully deed-restricted, no owner-use dilution) has a blended VMT/capita at or below the threshold.
Market-rate ADUs reduce VMT: unrestricted market-rate ADUs lower VMT/capita versus an all-SFH project, but not enough — even at 100% inclusion of the best category — to cross the threshold.
Only deed-restricted ADUs reduce VMT: market-rate ADUs do not lower VMT/capita, but senior- or income-restricted ADUs do.
Calculator defaults
Owner-used / inert ADU share defaults to 12% (ABAG 2022 statewide survey) — adjust the slider if a better local estimate is available for the county you're evaluating.
Caveats
VMT/capita levels are Replica-derived and will not exactly match travel-demand-model-based official thresholds published by individual jurisdictions; treat this as a research/screening tool, not an official determination.
Small counties (e.g. Alpine, pop. ≈1,600) have thinner ADU-cohort samples and carry more uncertainty than large counties.