Deer–Vehicle Collisions as a Herd-Density Signal
Ask which Illinois counties have the most deer–vehicle crashes and the answer is Cook and the suburbs, because that's where the cars are. Ask the right question (which counties have the highest collision density, normalized for exposure) and the map inverts to the rural west, the same ground the harvest data calls deer country.
- On IDNR's standardized collision density index, the top counties are rural western deer country (Schuyler (1,759), Calhoun (1,261), Brown (1,099), Fulton (1,094)), not the urban counties that lead in raw crash counts.
- Urban counties sit at the bottom of the density index (Cook 18, DuPage 15) despite recording the most total collisions. The normalization is what makes it a deer signal, not a traffic signal.
- The statewide index rose through the herd-growth decades, peaked around 2006, and declined as herd-reduction management took hold, closely mirroring the harvest record, an independent corroboration.
IThe wrong question and the right one
Every autumn, Illinois outlets publish the counties with the most deer–vehicle crashes, and every year the list is led by Cook and the collar suburbs. That list is a map of vehicle miles, not of deer. The fix is the metric IDNR already maintains: a standardized deer–vehicle-accident density index, normalized for exposure, used as a long-run herd-abundance proxy. On that index the ranking inverts: Schuyler, Calhoun, Brown, and Fulton on top, all squarely inside the western deer belt.
| 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 |
Highest collision-density counties, 2024, versus the urban floor. Higher = more deer per unit of driving exposure. Source: IDNR deer–vehicle-accident density index, FDR analysis.
Cook County records the most deer collisions in Illinois and has among the fewest deer per mile driven. That paradox is the whole reason to normalize.
IIA 35-year abundance curve
Aggregated statewide, the density index traces the arc of the modern Illinois deer herd: it climbed through the 1990s and early 2000s, peaked around 2006, and has drifted down since: the same period the state pushed antlerless harvest to reduce density and contain CWD. That the collision index and the harvest record bend together, despite being collected by separate systems, is what makes each more trustworthy.
IIIHow strong is the signal?
Honesty about the limits sharpens the tool. In any single year, the cross-county correlation between the collision-density index and total harvest is only moderate — about r = 0.31 in 2024 across the 100 reporting counties. That is exactly what you would expect, and it is the argument for the method rather than against it: one season's collision tally is buffeted by weather, deer movement, and reporting completeness, just as one season's harvest is buffeted by permits and mast. The value is not in any single year's rank but in the 36-year normalized curve, where those idiosyncratic shocks average out and a genuine abundance signal remains, one that bends in step with the independent harvest record. Treat a single year's density rank as a hint; treat the multi-year trajectory as evidence.
IVWhy it matters
Harvest data measures deer that hunters killed, which depends on permits, access, weather, and effort. The collision density index has none of those filters: it accrues wherever deer and cars coexist, every day of the year, and it reaches back to 1989, sixteen years before our harvest series begins. For a county where harvest looks ambiguous, the collision curve is often the tiebreaker. And because it is a public record collected for an entirely different purpose, it is the closest thing Illinois has to a free, unbiased second opinion on where the deer actually are.
- The deer–vehicle-accident density index is a standardized herd-abundance proxy derived from Illinois crash records; IDNR uses collision density as a long-run population indicator. Raw statewide collision counts (led by urban counties) are reported separately by IDOT.
- Single-year cross-county correlation (density index vs. total harvest, 2024, n = 100): r ≈ 0.31 — moderate, as expected for one noisy season; the multi-year normalized series is the intended unit of analysis. 1996 is omitted from the statewide series (incomplete record).
Fully Drawn Research is an independent data analysis desk, not affiliated with the Illinois Department of Natural Resources, the Illinois Department of Transportation, or any mapping or outfitting provider. 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.