Crash Data

SWITRS Geocoding Overview

Learn how SafeTREC geocodes SWITRS crash records, the data sources we rely on, and the steps we take to ensure every crash is accurately placed on the map.

Reading time: 5 minutes

Introduction

Geocoding converts locations such as street addresses or intersections into geographic coordinates that can be displayed on a map. For SWITRS records incorporated into TIMS, SafeTREC assigns coordinates based on street descriptions and highway postmile details. These coordinates are included in every raw data download as the POINT_X and POINT_Y fields.

SWITRS also stores the GPS-based LATITUDE and LONGITUDE reported by officers in the field. These values are useful but are only used to map a small percentage of records that cannot be accurately geocoded by the SafeTREC workflow.

Data sources

A combination of authoritative datasets supports SWITRS geocoding:

Additional detail about TSN and CRS maps is available through the Caltrans Office of Highway System Information & Performance.

Geocoding workflow

SafeTREC performs SWITRS geocoding in several stages. The high-level process is shown in the diagram below, followed by more detail about how we handle state highway and local roadway crashes.

Flowchart illustrating the SafeTREC SWITRS geocoding workflow
SafeTREC's multi-step process for geocoding SWITRS crashes.

State highways

State highway crashes are geocoded with a linear-referencing workflow that measures the distance from known postmile markers. SafeTREC compares SWITRS fields with the state highway reference system to place crashes precisely along each route. The Caltrans Postmile System documentation describes the underlying reference network.

Critical SWITRS fields include ROUTE NUMBER, SIDE OF HIGHWAY, ROUTE SUFFIX, POSTMILE PREFIX, and POSTMILE. These values are provided by Caltrans after the original crash report is submitted.

Since 2015, the California Highway Patrol has used the California Collision Reporting System (CCRS) to submit crashes electronically. Because the new system does not return highway attributes to SWITRS, SafeTREC evaluates the accuracy of officer-supplied GPS coordinates and supplements them with our linear-referencing method when needed.

Local roadways

Local roadway geocoding begins with a custom address locator built from Streetmap Premium data in ArcGIS. The locator matches crash records to intersections using the PRIMARY ROAD, SECONDARY ROAD, COUNTY, and CITY fields.

When an intersection match is found, SafeTREC applies any officer-specified offsets to place the crash at the correct distance and direction from the intersection. These offsets allow us to portray non-intersection crashes accurately.

Assigning functional classification

After geocoding, SafeTREC assigns each crash a roadway functional class so agencies can analyze safety patterns by facility type. The Federal Highway Administration defines seven functional classifications:

  1. Interstates
  2. Other freeways or expressways
  3. Other principal arterials
  4. Minor arterials
  5. Major collectors
  6. Minor collectors
  7. Local streets

Caltrans distributes official functional class data through the California Road System maps, which SafeTREC integrates into TIMS. Users can explore the classifications in both the SWITRS Query & Map and the SWITRS GIS Map tools under the Infrastructure filter category.