Safety Score Methodology

Resumen

The DK Law Safety Index provides data-driven safety rankings for cities across California, helping residents, policymakers, and safety advocates understand relative traffic risks in their communities. Our methodology combines multiple crash indicators to create a comprehensive safety score that reflects both the frequency and severity of traffic incidents.

Study Parameters

Analysis Period: 2021-2023 (3-year aggregate)
Geographic Scope: Incorporated California cities with population ≥50,000
Data Window: Most recent complete three-year period with validated data
Update Frequency: Annual (pending data availability)

Data Sources

Our analysis relies exclusively on authoritative, publicly available government data sources:

Primary Data Sources

• California Highway Patrol SWITRS (Statewide Integrated Traffic Records System): Comprehensive database of all police-reported traffic collisions in California
• NHTSA FARS (Fatality Analysis Reporting System): National census of fatal traffic crashes, used for validation and supplemental detail
• U.S. Census Bureau ACS (American Community Survey 5-Year Estimates): Population baselines for per-capita calculations
• Caltrans Traffic Operations: Annual Average Daily Traffic (AADT) volumes and highway system data

Data Validation

All crash statistics undergo cross-validation between SWITRS and FARS databases to ensure accuracy, with discrepancies >5% flagged for manual review.

Scoring Methodology

Core Safety Metrics

Each city is evaluated across four key traffic safety dimensions:

1. Crash Frequency Rate

Formula: Total Crashes per 1,000 Residents = (Total Crashes 2021-2023 / City Population) × 1,000

What it measures: Overall crash exposure risk adjusted for population size

2. Severe Injury & Fatality Rate

Formula: KSI per 1,000 Residents = (Killed + Severe Injury Crashes / City Population) × 1,000

What it measures: Frequency of life-altering or fatal traffic incidents

3. Impaired Driving Index

Formula: DUI Percentage = (DUI-Involved Crashes / Total Crashes) × 100

What it measures: Prevalence of alcohol or drug-impaired driving as a crash factor

4. Vulnerable Road User Risk

Formula: Pedestrian/Cyclist Percentage = (Pedestrian or Cyclist-Involved Crashes / Total Crashes) × 100

What it measures: Safety risks for those walking and biking

Score Calculation

Each metric is normalized to a 0-100 scale using min-max scaling against statewide benchmarks:

Normalization Formula:
Score = 100 × (Max Value – City Value) / (Max Value – Min Value)

Note: Higher scores indicate safer conditions (inverse relationship with risk)

Composite Safety Score

The final Traffic Safety Score is calculated as the arithmetic mean of all four normalized metrics:

Composite Score = (Crash Frequency Score + Severe Injury Score + Impaired Driving Score + Vulnerable User Score) / 4

Scores range from 0 (least safe) to 100 (safest), providing an intuitive scale for public communication.

Statistical Adjustments

Winsorization

To prevent statistical outliers from skewing results, all metrics are winsorized at the 5th and 95th percentiles before normalization. This standard statistical practice ensures that unusual data points don’t unduly influence rankings while preserving genuine safety differences.

Minimum Sample Size

Cities with fewer than 50 total reported crashes across the three-year period are excluded from rankings to ensure statistical reliability.

Boundary Considerations

Crashes occurring on controlled-access highways (freeways) are excluded from city calculations when the highway passes through but is not managed by the city, ensuring fair comparisons between cities with and without major highways.

Ranking Methodology

Cities are ranked from safest to least safe based on their Composite Safety Score. In cases of tied scores, the following tiebreakers are applied in order:

1. Lower severe injury/fatality rate
2. Lower overall crash rate
3. Lower impaired driving percentage

Quality Assurance

Data Validation Process

• Cross-reference SWITRS and FARS databases for fatal crash counts
• Geocode verification to ensure accurate city boundary assignment
• Year-over-year consistency checks to identify data anomalies
• Manual review of top and bottom 10% of rankings

Exclusion Criteria

The following data are excluded from analysis:

• Non-injury property damage only collisions (where data incomplete)
• Crashes with invalid or missing location data
• Duplicate records identified through incident matching
• Crashes outside incorporated city boundaries

Important Limitations

Data Coverage

• Police-reported crashes may underrepresent minor incidents
• Reporting practices vary by jurisdiction and agency
• Bicycle and pedestrian incidents are particularly prone to underreporting

Population Baseline

• Per-capita rates use residential population, not accounting for commuter or visitor traffic
• Cities with high tourism or commuter populations may show different risk profiles
• Census estimates have inherent margins of error

Temporal Factors

• Three-year aggregation may mask recent safety improvements or deteriorations
• Seasonal variations are averaged across the study period
• Data processing delays mean rankings reflect conditions 12-18 months prior to publication

Interpretation Guidelines

What These Rankings Show

• Relative traffic safety performance across California cities
• Multi-dimensional view of traffic safety challenges
• Data-driven baselines for safety improvement initiatives

What These Rankings Don’t Show

• Absolute risk levels (all driving involves some risk)
• Neighborhood-level variations within cities
• Safety of specific roads or intersections
• Impact of recent infrastructure changes not yet reflected in data

Annual Updates

Rankings are updated annually as new validated data becomes available from state and federal sources, typically with an 18-month lag (e.g., 2024 rankings published in mid-2025 using 2021-2023 data).

Contact & Citations

For questions about methodology or data access, contact: [[email protected]]

Suggested Citation:
DK Law California Traffic Safety Index Methodology (2024). Retrieved from [URL]

Data Repository:
Aggregated datasets and supplementary materials available at: [URL]

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Last Updated: Nov 19, 2025
Version: 1.0

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