The Illinois Department of Transportation publishes an annual Illinois Traffic and Pedestrian Stop Study. This study provides valuable information about local police departments and may help to determine whether or not each department exhibits any racial bias in their traffic stop practices.
One of the major data points of this study is the "Rate Ratio vs. White". It is the stop ratio of each race category to that of whites. You will notice the white ratio is always 1. If the ratio is 2.0, for example, it means that the minority stop rate is twice the white rate.
One of the main numbers IDOT reports for these years is the "ratio": the minority share of stops divided by the minority share of the driving population. A ratio of 1.00 would mean minority drivers were stopped in proportion to their share of the drivers; a higher ratio means they were stopped more often than that share, and a lower one means less often.
IDOT's benchmark population for this department is people. If this community were represented by dots, it would look like this:
| Race | Total | Percent | |
|---|---|---|---|
| White | |||
| Black | |||
| Hispanic |
Here is what our dots would look like if they matched the racial makeup of the actual stop results for this municipality:
| Race | Total | Percent | |
|---|---|---|---|
| White | |||
| Black | |||
| Hispanic |
stops were reported by this municipality in . In our scaled-down municipality of people, we would have stops. If those stops reflected the racial makeup of the larger municipality, they would look like this:
The actual racial representation of stops for this municipality according to this report looked like this:
IDOT's report doesn't count this department's population by race. Instead it estimates that of the drivers in the department's area are minority drivers (everyone who is not White). If that area were represented by dots, it would look like this:
In this department made traffic stops where the driver's race was recorded, and of them were minority drivers. Here are dots matching the racial makeup of those stops:
How similar are Figure 1 and Figure 2? The more similar they are, the closer this department's stops came to matching its estimated driving population.
The degree to which racial bias exists in this municipality, according to the data of this study, is reflected in the above charts. How similar are Figure 3 and Figure 4? Figure 1 and Figure 2? The more similar they are, the less racial bias is presented by the data. The more different they are, the more racial bias is presented by the data.
How 's stops have changed from to . Select a point to jump to that year. Hollow points are based on fewer than stops, so they can swing widely. A gap means the department did not report that year.
A value of 1.0 means a group was stopped at the same rate as White drivers; 2.0 means twice the rate. Only years with a population benchmark by race can be shown.
| Year | Stops with a recorded race | Minority share of stops | Minority share of population | Ratio of the two | Black stop rate vs. White | Hispanic stop rate vs. White |
|---|
This study contains much more data than presented in these graphs. I chose to select the simplest set of data which I believe is the easiest to understand and which does the best job of summarizing the outcomes of the study - namely the "Rate Ratio vs White". Please feel free to visit the study at the link above for more detailed information.