Safety Score Methodology

Last Updated: August 2026
Version: 3.0

Overview

The DK Law Service Area Traffic Safety Index provides data-driven safety rankings for 21 California cities within DK Law’s service areas. Our methodology combines multiple crash indicators from the California Office of Traffic Safety (OTS) to create a composite score reflecting traffic safety conditions in each community.

Study Parameters

Data Years: 2021, 2022, and 2023, pooled 

Geographic Scope: 21 California cities within DK Law service areas 

Data Source: California Office of Traffic Safety

Update Frequency: Annual, as OTS releases new data years

Cities Included

Anaheim, Bakersfield, Buena Park, Cerritos, Costa Mesa, Fresno, Fullerton, Irvine, La Mirada, Long Beach, Los Angeles, Moreno Valley, Newport Beach, Rancho Cucamonga, Riverside, Sacramento, San Diego, San Francisco, San Jose, Victorville, Whittier.

Data Source

All crash statistics are sourced exclusively from the California Office of Traffic Safety (OTS) Crash Rankings database. OTS compiles crash counts from SWITRS (Statewide Integrated Traffic Records System), population estimates from the California Department of Finance, and daily vehicle miles traveled from Caltrans. OTS provides consistent, publicly verifiable data across all California cities, and every figure in this index traces to a specific OTS city-year page.

Three Years of Data

Earlier versions of this index scored each city on a single data year. For the largest cities, that works – San Diego’s impaired-driving share varied by less than one percent across four consecutive years. For the smallest, it does not. A city of roughly 48,000 residents may record only a handful of alcohol-involved victims in a given year, and a shift of two or three victims moves its score substantially. In cities under 200,000 residents, we measured a median year-over-year swing of 1.6x in impaired-driving share, against 1.1x for cities above 200,000.

Pooling three years addresses this. Victim counts and population are summed across 2021, 2022, and 2023, and the ratios are calculated from those totals. We do not average three annual percentages, which would give a light year the same weight as a heavy one and reintroduce the problem we are correcting for.

Scoring Methodology

Core Safety Metrics

Each city is evaluated across three traffic safety dimensions. All three use victims killed and injured – the figure OTS publishes. OTS does not publish a plain “total crashes” count, so no metric in this index is crash-based.

1. Crash Rate Score (40% Weight)

Formula: Crash Rate per 1,000 = (Total Fatal and Injury Victims ÷ Population) × 1,000. Both figures are pooled across 2021–2023, using the OTS “Population (Avg)” field.

What it measures: Overall crash exposure adjusted for population size – the likelihood that a resident is involved in a crash resulting in death or injury on the streets under city jurisdiction.

Why 40% weight: It is the broadest measure available and the only one capturing crash frequency on a per-capita basis.

2. Impaired Driving Score (35% Weight)

Formula: DUI Percentage = (Alcohol-Involved Victims ÷ Total Fatal and Injury Victims) × 100. Both figures are pooled across 2021–2023.

What it measures: The share of crash victims in incidents where a driver, pedestrian, or bicyclist was classified by the reporting officer as having been drinking.

Why 35% weight: Impaired-driving crashes tend to be more severe and are highly preventable.

3. Vulnerable Road User Score (25% Weight)

Formula: VRU Percentage = ((Pedestrian Victims + Bicyclist Victims) ÷ Total Fatal and Injury Victims) × 100. All figures pooled across 2021–2023.

What it measures: The share of crash victims who were on foot or on a bicycle – the road users who face the most severe outcomes.

Why 25% weight: It describes crash composition rather than overall frequency.

Score Calculation

Each metric is normalized to a 0–100 scale using min-max scaling within the 21-city dataset:

Normalisation Formula: Score = 100 × (Maximum − City Value) ÷ (Maximum − Minimum)

Higher scores indicate safer conditions. The relationship is inverse: a city with the highest impaired-driving share scores 0 on that metric, and a city with the lowest scores 100.

Composite Safety Score

Composite Score = (Crash Rate Score × 0.40) + (DUI Score × 0.35) + (VRU Score × 0.25)

Scores range from 0 to 100.

Statistical Adjustments

Winsorization

To prevent outliers from dominating the scale, each metric is winsorized at the 5th and 95th percentiles before normalization.

With 21 cities, the 5th and 95th percentiles fall precisely on the second-lowest and second-highest city values. This has a visible consequence worth stating plainly: on each metric, two cities will share a score of 100 and two will share a score of 0. 

These are not ties in the underlying data and not errors. The lowest-scoring city on a metric has been brought up to the second-lowest value, and the highest brought down to the second-highest.

Ranking Tiers

Cities are classified into four tiers based on their composite ranking:

Tier	排名	Classification
Tier 1	1-5	Safest
Tier 2	6-11	Moderate
Tier 3	12-16	Concerning
Tier 4	17-21	Highest Risk

Ranking Methodology

Cities are ranked from safest to least safe by composite score. Where scores tie, the following tiebreakers apply in order:

1. Lower crash rate per 1,000 residents
2. Lower impaired driving percentage
3. Lower vulnerable road user percentage

Important Limitations

Jurisdictional Scope

This is the most significant limitation of any index built on OTS city data, and it affects every city here.

OTS city rankings cover local streets and those state highways where the city shares jurisdiction with the California Highway Patrol. They exclude freeways and state highways outside city jurisdiction. County rankings include them. Per-capita crash rates therefore understate exposure in cities where a large share of driving happens on freeways, and are comparatively less forgiving of cities where most travel is on surface streets.

Los Angeles is the clearest illustration. The city contains 38.8% of Los Angeles County’s population but accounts for 13.8% of the county’s fatal-and-injury victims, because so much Los Angeles driving occurs on freeways that fall outside city jurisdiction. Read the Crash Rate Score as “fatal and injury victims on city-jurisdiction streets, per resident” rather than as total road risk.

For corroboration that this treatment is consistent with the source: OTS’s own 2023 rankings, which weight crash counts against both population and vehicle miles traveled using an Empirical Bayesian method, independently place Los Angeles 15th of 15 – the best position – among comparably sized California cities for total fatal-and-injury victims. Across the eleven largest cities in this index, our Crash Rate Score and OTS’s own ranking agree at a rank correlation of 0.97.

Sample Size in Smaller Cities

Pooling three years substantially improves stability, but in the smallest cities the counts remain modest. Cerritos records 12 alcohol-involved victims across the full three-year period and La Mirada 25. These are the smallest numerators in the index and their impaired-driving scores should be read as indicative rather than precise.

Data Coverage

• OTS reports fatal and injury victims as a combined figure; isolated fatality rates are not available
• The 2021 data year still reflects some post-pandemic distortion in traffic volumes and travel patterns. It is included because sample size matters more for the smaller cities in this index than perfect behavioral comparability across years
• Police-reported crashes understate minor incidents
• Pedestrian and bicycle incidents are particularly prone to underreporting
• OTS notes that its rankings are indicators of potential problems, and that local circumstances may overstate or understate any given figure

Normalization Scope

• Rankings are normalized within these 21 cities only, not against statewide benchmarks
• A city scoring 50 sits mid-range for this dataset, not for California

Population Baseline

• Per-capita rates use the OTS “Population (Avg)” field, which OTS sources from California Department of Finance estimates
• Resident population does not account for commuter or visitor traffic. Cities with large daytime populations relative to their resident base – Newport Beach and Anaheim in particular – carry more road exposure than their population figure implies
• Population estimates have inherent margins of error

Interpretation Guidelines

What These Rankings Show

• Relative traffic safety performance across DK Law service areas
• A multi-dimensional view combining exposure, impairment and vulnerable road user risk
• A three-year baseline less sensitive to single-year fluctuation than annual data

What These Rankings Don’t Show

• Absolute risk levels
• Risk on freeways and state highways outside city jurisdiction
• Neighborhood-level variation within cities
• Safety of specific roads or intersections
• Isolated fatality rates

Annual Updates

Rankings are updated as OTS releases new data years. OTS typically publishes a given year’s rankings 12–18 months after that year ends, but 2024 data has not yet been released as of July 2026, a longer gap than usual. As of this writing, the most recent OTS data year remains 2023. When 2024 becomes available, the index will roll forward to a 2022–2024 pooled window.

This methodology has been reviewed by independent traffic safety experts and follows established practices in transportation safety research and epidemiological analysis.