Chicago Police Department versus other Illinois Law Enforcement Agencies

Correlation with the Veil of Darkness

By: Tiasia Saunders

Description of the traffic — An illustrative image of a police officer monitoring traffic activity. Photo courtesy of Adobe Stock.

Overview

Using 2024 Chicago Police Department and other Illinois Law Enforcement traffic data, this project examines whether racial disparities extend beyond CPD into suburban and downstate agencies. While CPD has faced scrutiny, especially after the fatal shooting of Dexter Reed, smaller agencies operate with far less oversight and accountability reporting. This analysis uses a veil of darkness lens to explore potential racial disparities.

Veil of Darkness Explanation

It is a statistical methodology created by Jeffrey Grogger and Greg Ridgeway (2006) to test for racial bias in traffic stops.
It compares daytime vs nighttime stops, based on the idea that darkness masks driver race.
If bias exists, minority drivers should be stopped more during daylight hours when race is visible.
Time definitions:
- Daytime: 6 a.m. – 5 p.m.
- Nighttime: 10 p.m. – 4 a.m.
- Transition hours excluded: 4–6 a.m. and 5–10 p.m.

Hypothesis / Potential Problems

I suspect racial disparities in stop patterns exist across Illinois agencies, not just CPD.
Stops and searches may differ by race depending on visibility conditions.
The veil of darkness method may be imperfect because it uses fixed time ranges instead of actual sunrise/sunset data.

Data Origin / Table Definitions

Data comes from the Investigative Project on Race and Equity (2021–2024 traffic and pedestrian stops data received through a FOIA request).
Includes Chicago Police Department and other Illinois law enforcement agencies.
Population data from the 2020 U.S. Census is used for per-capita calculations.
Sources:
- Illinois Population Data
- Chicago Population Data

The Why

If disparities exist across all Illinois agencies, it suggests biased enforcement is not isolated to Chicago and may require statewide accountability reforms.

Traffic Stop Timelines

Chicago Police Department Timeline

Tracks CPD traffic stops by month (2024)
So, I looked into an explanation as to why there was a steady decrease of traffic stops by the CPD in 2024, so once looking it up I found this article, which explained that there were 210,622 undocumented traffic stops in 2024, despite a 2003 law requiring them to document every traffic stop.
In addition, the Dexter Reed incident in which a 26-year old Black man died at the result of a shootout incident between him and policemen in late March of 2024, may have resulted in heavier scrutiny due to advocacy groups and civil rights lawsuits.
The incident had stemmed from a seatbelt violation traffic stop.

Other Illinois Agencies Timeline

Tracks statewide law enforcement agency comparison
There are 786 different law enforcement agencies in the data.
The decline in June could be related to the 200,000 undocumented traffic stops and on this day Chicago Police Superintendent announced at a press release the decrease in traffic stops.

Per-Capita Traffic Stop Analysis

Chicago Police Department

Shows stop rates adjusted for racial population size which helps identify disparities hidden by raw counts
When accounting for race population size, it shows me that Native Hawaiian and Pacific Islander (NHPI) drivers had the highest per capita rate of traffic stops in 2024, with 32,083 stops per 100,000 residents, but this is mainly due to the small NHPI population size.
Black drivers had the second highest per capita rate with 17,088 stops per 100,000 residents, followed by Hispanic drivers with 12,492 stops per 100,000 residents.
White drivers had the lowest per capita rate of 4,912 stops per 100,000 residents.
Asian drivers had a per capita rate of with 7,052 per 100,000 residents.
American Indian drivers had a per capita rate of 6,610 stops per 100,000 residents.

Other Illinois Law Enforcement Agencies

Illinois law enforcement per capita traffic stop rates by race

Compares statewide traffic stop of hundreds of law enforcement agencies
Illinois state traffic stop rates were highest among NHPI and American Indian residents, though those rates are based on much smaller population counts.
Among larger racial groups, Black residents had the highest stop rate, with about 21,373 stops per 100,000 residents, compared with 12,779 per 100,000 white residents.
Following that, Hispanic residents made up 12,779 stops per 100,000 reesidents, while Asian had the lowest per capita with 9,415 traffic stops of 100,000 residents.

Chicago Police Department Heatmap

License plate and registration violations were the most common reason for traffic stops.
NHPI drivers were the only group where moving violations were the largest category (40.2%).
Asian drivers had the highest proportion of equipment-related stops (37.8%).
Stop patterns vary across racial groups, suggesting uneven enforcement experiences.

Illinois Law Enforcement Agencies Heatmap

Moving violations dominate statewide stops across all racial groups.
Asian and AIAN drivers show the highest share of moving violation stops (~69%).
Black drivers have the highest share of registration-related stops (28.3%).
Statewide enforcement differs from Chicago, with stronger emphasis on moving violations.

Chicago Search Rates

Black drivers had the highest search rate at 1.82% after stops.
Hispanic and NHPI drivers also appear in the top three search rates.
White and Asian drivers had the lowest search rates (below 0.3%).
Search disparities suggest uneven post-stop enforcement outcomes.

Illinois Search Rates

Black drivers experienced the highest search rate at 7.04%, more than double the rate of Hispanic and White drivers, indicating a significantly higher likelihood of searches following traffic stops.
Hispanic (3.95%) and White (3.44%) drivers had similar search rates, though both were substantially lower than Black drivers in this dataset.
NHPI drivers had a moderate search rate at 3.02%, but this is based on a much smaller total number of stops, which can make rates more sensitive to variation.
Asian and American Indian drivers had the lowest search rates at 1.26% and 1.88% respectively, suggesting fewer searches following traffic stops compared to other racial groups.

Veil of Darkness Analysis

Chicago Police Department

Black drivers accounted for the largest share of Chicago traffic stops during both daytime (48.0%) and nighttime (41.0%) hours. Additionally, it followed the suspected pattern of veil of darkness through the traffic stop percentage dropping by 7% at nighttime.
Hispanic drivers' share increased from 31.7% during the day to 37.0% at night.
White drivers accounted for 16.2% of daytime stops and 13.0% of nighttime stops, which follows a veil of darkness lens through police not recoginizing the drivers race at nighttime, but it descreased by 3.2%.
The racial distribution of stops in Chicago differs substantially from statewide Illinois traffic-stop patterns, where White drivers make up the majority of stops.

Illinois Law Enforcement Agencies

Statewide agencies show similar but more distributed stop patterns.
Black drivers accounted for 18.8% of daytime traffic stops but 27.1% of nighttime stops, representing an increase of 8.3 percentage points (about a 44% increase in stop share at night).
White drivers accounted for 58.1% of daytime stops but dropped to 46.8% at night, representing an 11.3 percentage point decrease (about a 19.5% decline in stop share after dark).
Hispanic drivers also saw an increase in stop share, rising from 18.4% during daytime hours to 20.8% at night, while Asian, AIAN, and NHPI stop shares remained relatively stable across lighting conditions.
These findings do not align neatly with the traditional Veil of Darkness expectation that minority drivers would account for a smaller share of stops when visibility is reduced. Instead, Black and Hispanic drivers represented a larger share of nighttime stops, suggesting that other factors beyond driver visibility may influence statewide traffic stop patterns.
Because this analysis uses fixed daytime and nighttime categories rather than actual sunrise and sunset times, these results should be interpreted cautiously and viewed as descriptive rather than causal evidence against or for the Veil of Darkness framework.

Illinois Agencies Pattern

Statewide agencies show similar but more distributed stop patterns.
Disparities are present but vary more across departments than CPD alone.
Suggests enforcement differences are not isolated to Chicago.

Key Takeaway

The Veil of Darkness comparison suggests racial disparities may be present across multiple Illinois jurisdictions.
However, results are descriptive and depend on time-based assumptions rather than exact visibility conditions.
Further refinement using sunrise/sunset data would strengthen the analysis.

Conclusion:

CPD Traffic Stops (Racial Breakdown): Black drivers represented the largest share of traffic stops during both daytime and nighttime hours, accounting for 48.0% of stops during the day and 41.0% at night. Hispanic drivers’ share increased from 31.7% during the day to 37.0% at night, while White drivers accounted for 16.2% of daytime stops and 13.0% at night.
Illinois Law Enforcement (Non-CPD Agencies): Black drivers represented a larger share of traffic stops at night (27.1%) compared to daytime (18.8%), while White drivers’ share declined from 58.1% during the day to 46.8% at night, suggesting a shift in stop composition after dark.
Per-Capita Calculations: Across CPD data, NHPI and American Indian residents show the highest per-capita stop rates, though these figures are influenced by small population sizes. Among larger racial groups, Black residents had the highest stop rate at approximately 7,374 stops per 100,000 residents, compared to about 580 per 100,000 White residents. In statewide Illinois law enforcement data, a similar pattern emerges, with Black residents again experiencing the highest stop rates among major racial groups.
Overall Interpretation: The results provide partial support for the Veil of Darkness hypothesis. While the CPD data analysis highlights the decrease in Black drivers arrest rate at nighttime, the Illinois statewide data highlights an increase in the traffic stops. There is a noticeable increase in the share of stops involving Black drivers at night and a decrease in the share involving White drivers, suggesting potential visibility-related disparities. However, these findings are not conclusive and may also reflect other factors such as driving patterns, policing strategies, or geographic differences.
Limitations: This analysis relies on simplified time-based definitions of daylight and nighttime rather than precise sunrise and sunset data, which may limit the accuracy of the Veil of Darkness measure.

How I Could Expand This Project

I could improve the Veil of Darkness methodology by incorporating actual sunrise and sunset times throughout the year using weather or astronomical datasets, making the analysis more precise than fixed time blocks.
I could expand the analysis to include search hit rates by race and reason for stop, which would help determine whether post-stop outcomes differ across racial groups.
I could also examine whether certain reasons for stops are disproportionately associated with searches or citations, which would further clarify how enforcement decisions vary across racial groups.
I could look at traffic stops through a georgraphic lens, car type, and additional factors.