Cook County and other counties with heavy traffic lead Illinois' unadjusted deer-crash totals because they have more cars. Adjusting collision density for exposure moves the leaders to rural western Illinois, where deer harvest totals are also high. Those totals, though, vary with hunting effort, permit supply and land access.
KEY FINDINGS
- The leaders in IDNR's standardized collision density measure lie in rural western deer country: Schuyler at 1,759, Calhoun at 1,261, Brown at 1,099, and Fulton at 1,094. Urban counties lead the unadjusted crash totals instead.
- The index places urban counties at its lowest end (Cook 18, DuPage 15), even though they record the highest total crash counts. Dividing collisions by driving distance adjusts for traffic volume when treating crashes as an indicator of deer abundance.
- Across Illinois, the index increased during the 1990s, continuing into the early 2000s. It reached its high point around 2006 before dropping, following the harvest history closely.
IThe wrong question and the right one
Crash rates adjust for distance traveled
Cook County has led Illinois in IDOT's unadjusted deer-crash totals: 493 for 2021, compared with 452 in Madison and 326 in Will. Suburban counties and those in metropolitan areas also rank high. That ranking reflects how many miles vehicles travel. IDNR already tracks a standardized deer-vehicle-accident density index, calculated as deer crashes for each billion miles traveled by vehicles. The agency uses it as a proxy for herd abundance over the long term. Adjusting for driving distance reverses the ranking. Schuyler, Calhoun, Brown and Fulton lead the index, all firmly within western Illinois' deer belt.
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| Rank | County | DVA density index, 2024 |
|---|---|---|
| 1 | Schuyler | 1,759 |
| 2 | Calhoun | 1,261 |
| 3 | Brown | 1,099 |
| 4 | Fulton | 1,094 |
| 5 | Crawford | 1,042 |
| · | Cook (urban floor) | 18 |
| · | DuPage (urban floor) | 15 |
Cook County's deer-crash total leads Illinois, yet its rate relative to miles traveled ranks among the state's lowest (density index 18). That gap explains why the adjustment matters.
IIA 35-year abundance curve
The herd's climb, crest and drop appear here
For Illinois as a whole, the density index rose through the 1990s and early 2000s, hit its highest point around 2006, and has gradually fallen since. After that peak, IDNR established a collision-rate target for each county and increased hunting permits in counties with rates above their targets. The similar direction of the collision index and harvest record gives the trend more support. Those records are connected, though, because IDNR increases permits in response to high collision rates.
IIIHow strong is the signal?
Decades give a dependable signal despite yearly noise
Looking across counties in any one year, the correlation between the collision-density index and total harvest is moderate: about r = 0.31 in 2024 among the 100 reporting counties. That relationship by itself cannot establish a population trend. A season's collision count varies with crash reporting and deer movement, particularly during the October to December rut peak, while a season's harvest varies with permits. The useful evidence comes from the normalized curve across multiple years: 35 annual values from 1989 to 2024, excluding 1996. Smoothing the yearly fluctuations makes the trend easier to see, and the curve follows the same direction as the harvest record. A density ranking from one year offers a clue; the longer trend in that normalized curve provides evidence.
IVWhy it matters
Another no-cost way to check deer abundance
Harvest totals count the deer killed by hunters and depend on permits, access, weather, and hunting effort. The collision density index is shaped by a different set of factors: it does not rely on hunter permits, access, or effort, but traffic volume, crash reporting, and the peak in deer movement from October to December do shape it. The index goes back to 1989, sixteen years earlier than the start of our harvest series. When a county's harvest data leaves the interpretation uncertain, the collision curve often settles the call. Drawn from public records gathered for another purpose, it also gives Illinois a second, no-cost way to check relative deer abundance.
NOTES AND SOURCES
- Illinois crash records provide the basis for the standardized deer-vehicle-accident density index, a proxy for herd abundance. IDNR treats collision density as an indicator of population over the long term. Urban counties lead the unadjusted statewide collision totals, which IDOT reports separately.
- Across counties in 2024, the density index and total harvest have a moderate single-year correlation (n = 100): r ≈ 0.31. That fits expectations for a noisy season. Analysis is meant to focus on the normalized sequence across years. An incomplete record leaves 1996 out of the statewide sequence.
Fully Drawn Research is an independent data analysis desk, not affiliated with any state wildlife agency, transportation agency, mapping provider or outfitter. The density index is a relative abundance proxy, not a deer census, and is presented for informational use only, not driving-safety, insurance, hunting, or investment advice. Figures current to 2024.
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