Frequently Asked Questions

Common questions about the data, the dashboards, and how VisZero works.

All data is sourced live from the City of Chicago Open Data Portal, which is maintained by the Chicago Department of Transportation (CDOT). Two datasets are used:
  • Chicago Traffic Fatalities — the Traffic Crashes – People dataset, which records individual crash victims.
  • Chicago Traffic Crashes — the Traffic Crashes – Crashes dataset, which records crash events including cause and injury count.
VisZero does not store or cache data. Every page load queries the source directly.

Chicago Traffic Fatalities focuses on individual people involved in fatal or serious injury crashes. Each record represents one person (a cyclist, pedestrian, driver, etc.) and you can filter by their role in the crash.

Chicago Traffic Crashes focuses on crash events themselves. Each record represents one crash and includes the primary contributory cause (distraction, speed, impairment, etc.), crash type, and total injuries. This dataset covers all reported crashes, not just fatal ones.

There are two common reasons:

  1. API result limit: The Socrata API returns up to 1,000 records per request. For long date ranges with many crashes, the result set is truncated. Try narrowing the date range or applying additional filters to see more complete data.
  2. Missing coordinates: Some crash records in the source data do not have precise geolocation. These records are included in the chart totals but cannot be plotted on the map.

The cause groups on the Chicago Traffic Crashes dashboard map to specific values in the source dataset's prim_contributory_cause field. Hover over each group label on the dashboard to see exactly which causes are included. A summary:

  • Distraction — cell phone use, texting, in-vehicle and outside-vehicle distractions
  • Speed — exceeding speed limit, unsafe speed for conditions, failing to slow to avoid a crash
  • Signals / signs — disregarding traffic signals, stop signs, yield signs, or road markings
  • Impairment — alcohol or drug influence, physical condition of the driver
  • Right of way — failure to yield, improper lane use, improper passing or turning, following too closely
  • Environment — weather, obscured vision, road defects or construction, animals
  • Vehicle / driver — equipment failure, erratic or reckless operation, lack of driving skill
  • Other / unknown — unable to determine, not applicable, miscellaneous causes

The City of Chicago updates both datasets on an ongoing basis, typically within a few days of a crash being reported and processed by CDOT. Because VisZero queries the API live on every request, the data you see is always as current as the source allows. There is no additional delay introduced by VisZero.

The heat map uses Mapbox GL JS to display crash density across Chicago. Warmer colors (red/orange) indicate areas with a higher concentration of crashes matching your current filters. The map reflects only the records returned by the current query — changing filters or the date range will update both the map and the chart.

VisZero does not currently offer a data export feature. To download the underlying data directly, visit the City of Chicago Open Data Portal: Both datasets can be exported as CSV, JSON, or accessed via the Socrata API.