Rate & confidence intervals

Every compute response reports a rate with a 95% confidence interval, plus the numerator and denominator it was built from.

The rate

rate = N / (D_miles / 1_000_000)

• N: the weighted count of crashes matching your selections (outcome, vehicle class, road type, weather, light, crash year, and so on). If under-reporting adjustment is on, Blincoe multipliers are applied here.
• D_miles: calibrated vehicle-miles travelled for the selected road groups and vehicle classes. D_billions is the same value in billions.
• rate: incidents per million miles (IPMM).

Numerator details

• Outcome is a nested filter: each outcome (observed_any_injury, airbag, ka, fatal, and the rest) is a subset of police_reported.
• Under-reporting: the adjusted option applies Blincoe et al. 2023 (NHTSA) multipliers, PDO ×2.48, non-fatal injury ×1.47, fatal ×1.0.
• Crash year: geofence cities carry a crash_year column. Selecting multiple years pools their crashes and scales the denominator by the number of years, so the rate stays an annual mean.

Denominator details

• Calibrated VMT (default) blends state-DOT roadway-inventory totals with FHWA VM-4 per-class fractions.
• denominator_vmt (California only) toggles between the Caltrans CPRD total and an HPMS functional-class total.
• multiplier_vmt selects the exposure source inside the operator-weighting multiplier. It is relevant only when operator_weighting=robotaxi, and ignored when operator_weight is set explicitly.

Operator weighting (geofence)

When operator_weighting=robotaxi, the rate is paired with a Chen 2025 spatial multiplier that re-weights region exposure toward where an ADS operator actually drives, using Waymo rider-only mileage. It applies only when every selected road type is a surface street. rate_non_dyn is always returned for reference. rate_dyn and multiplier are null when the operator mileage layer is not loaded.

The applied multiplier can be overridden with operator_weight, a positive number. When set, the headline becomes rate_dyn = rate_non_dyn × operator_weight and the CI scales by it, substituting a custom operator weight for the computed Waymo default. Leave it unset (null) to use the computed value. The override takes effect only on the same dynamic path: operator_weighting=robotaxi, surface-only, operator mileage present.

Confidence intervals

Routes and depots return a ci_method choice:

• Fay-Feuer (default): a weighted-Poisson interval with a phantom-event correction for zero-crash legs.
• Empirical Bayes: a Gamma-Poisson interval using between-cell rate variance.

Geofence intervals use the same weighted-Poisson machinery. rate_low and rate_high are the 95% bounds, and route responses also expose the propagated variance.

Data & licensing

Outputs are derived statistics only (rates, counts, bounds). The API never redistributes raw third-party crash records. Crash data comes from public state sources (TxDOT CRIS, CHP/CCRS SWITRS, ADOT ALISS, Nevada DOT, IDOT), exposure from FHWA HPMS and state DOT inventories, weather from NOAA ISD, and ADS exposure from the Waymo Dynamic Benchmark.

Litigation caveat

Safety data compiled for federal highway-safety programs is shielded under 23 U.S.C. § 409 in many states. Do not market these outputs as litigation evidence, and do not use crash data to solicit involved parties.

Attribute every published figure. Full per-source citations live in the project's data license register.