Brief for the Iowa Department of Transportation

The most economical fix for this corridor — not another warning sign

Prepared by the Pause for Paws student team · Waukee, Iowa · pauseforpawsusa.org

We built a public map of Iowa's vehicle-animal crash corridors from your own crash records, refreshed twice a month, at pauseforpawsusa.org. This brief asks for something more specific than awareness: a look at which fix is the most efficient and affordable one for the corridor below — fencing and a crossing, a lower-cost surface treatment, or some combination — starting with an engineering feasibility study.

Other corridors we've run this same analysis for are in the register at pauseforpawsusa.org — happy to send a version of this brief for any of them.

What doesn't reliably work

• Static wildlife-crossing signs, placed generically rather than at a specific time and place, show no measurable reduction in collisions.
• Real-time driver alerts (including our own app) raise awareness but don't reliably change driving behavior — drivers brake for an animal they see, not for a notification.
• Asking drivers to slow down without changing the road's design speed causes some to comply and most not to, mixing fast and slow traffic on the same road. That's a documented cause of more head-on collisions from impatient passing — the opposite of the intended effect.

What does

• Wildlife fencing paired with an underpass or overpass reduces collisions with large mammals by 80–100% when built together.
• Fencing alone displaces collisions to where the fence ends. Crossing structures alone, without fencing, do not reduce collisions.
• Structure type and dimensions need to match the target species — for example, elk, moose, and deer typically need wide overpasses, while smaller mammals will use narrower underpasses.
• Fencing also has a second function: guiding animals toward the crossing structure, which increases its use.

Fencing funnels, it doesn't block

Fencing alone is a barrier — that's a fair objection, and it's exactly why we're not asking for fencing alone. A fence with no crossing structure just moves the collision problem to wherever the fence ends, and it can cut a local population off from habitat on the other side. The fence's job is to funnel animals to a crossing they'll actually use, not to wall them out — which is why every figure in this brief assumes fencing and a crossing structure are built together, never one without the other.

How we sized the fence

Fence length isn't a guess. Research reviewed by Iowa State's InTrans Deer-Vehicle Crash Countermeasure Toolbox found that crossing/fencing installations kept losing animals around the fence ends, and recommends running the fence about 1/2 mile (0.8 km) past the edge of the high-crash area on each end. We measured "the high-crash area" directly: the actual spread of crash points assigned to each corridor by our clustering (same 4 km grouping the map uses), then added that half-mile buffer on both ends.

Suggested length = (span of crash points in the corridor) + 1 mile (two 0.5-mile end buffers). This is a data-driven starting point for a feasibility study, not a final alignment — the actual fence line still needs a site survey for property lines, drainage, and terrain.

Corridor	Crashes (2026)	Risk	Suggested fence length	Fencing, est.	+ 1 underpass, est.
IA010 — US 61, Louisa Co.	15	Highest	5.7 mi	$230K–$460K	+$500K–$2M
IA017 — US 30, Story Co.	13	Highest	5.2 mi	$210K–$420K	+$500K–$2M
IA008 — US 30, Story Co.	16	Highest	4.8 mi	$190K–$380K	+$500K–$2M
IA018 — US 6, Johnson Co.	13	Highest	5.0 mi	$200K–$400K	+$500K–$2M
IA020 — I‑80, Polk Co.	12	Highest	4.9 mi	$200K–$390K	+$500K–$2M

Fencing estimate: $40K/mile (one side) to $80K/mile (both sides), from published U.S. 8-ft game-fence contractor pricing — a materials-and-install ballpark, not a DOT bid; state highway contracts typically run higher once engineering, mobilization, and right-of-way are added. Underpass: a mid-size culvert-style crossing, not a multi-lane overpass (those run $5M–$15M+ and are usually only justified for elk/moose-scale species, which isn't Iowa's problem — ours is almost entirely white-tailed deer). Full corridor register at pauseforpawsusa.org; ask us for the other 21 corridors we've run this calculation for.

The full range of options, not just fencing

We also looked at a fencing-free idea getting attention lately: a road project in a country in the Asian subcontinent used a bright red, high-friction surface through a wildlife reserve, meant to trigger an instinctive slowdown — no barrier, nothing blocking animal movement. The closest U.S. product is High-Friction Surface Treatment (HFST), already used at dangerous curves nationwide. Its grit is usually calcined bauxite, an imported ore baked at 1,000–1,500°C for hardness — most of why it's expensive. State DOT research has tested cheaper substitutes that skip the import and the baking:

• Taconite — leftover rock from Minnesota's iron mines, being field-tested by MnDOT right now on real roads near Duluth: ~6% less grip than bauxite, no faster wear over time.
• Steel slag — a steelmaking byproduct, lab-tested alongside calcined bauxite for friction surfacing through Purdue's Joint Transportation Research Program, sponsored by Indiana DOT and FHWA (Yu et al., Journal of Materials in Civil Engineering, 2019).
• Flint chat — leftover chert from old lead-zinc mining in the Missouri–Oklahoma–Kansas tri-state district, tested in Oklahoma DOT-funded research.

None of the three alternatives has a published commercial price yet, so we use one conservative shared estimate for them ($18–26/sq yd vs. bauxite's sourced $25–35/sq yd). Cost also depends heavily on how much of the corridor gets treated: a full-length solid ribbon, or a shorter, patchy, rumble-strip-style application. Here is the complete comparison for this corridor (Louisa County, US 61, 5.7 mi), fencing + underpass against all four surface materials at every coverage level:

Coverage	Bauxite	Taconite *	Steel slag *	Flint chat *
100% (solid ribbon, full length)	$2.0M–$2.8M	$1.4M–$2.1M	$1.4M–$2.1M	$1.4M–$2.1M
50% (patchy, half the length)	$1.0M–$1.4M	$722K–$1.0M	$722K–$1.0M	$722K–$1.0M
25% (patchy, a quarter)	$502K–$702K ↓	$361K–$522K ↓	$361K–$522K ↓	$361K–$522K ↓
10% (patchy, a tenth)	$201K–$281K ↓	$144K–$209K ↓	$144K–$209K ↓	$144K–$209K ↓
For comparison: fencing + 1 underpass	$728K–$2.46M, one fixed cost — not something you dial up or down by coverage

↓ = cheaper than fencing + underpass even in the worst case for the surface and the best case for fencing. * Taconite, steel slag, and flint chat have no published commercial price yet, so they share one estimate (marked with *) rather than three invented, different numbers. Full 26-corridor range and a live, adjustable version of this table are at pauseforpawsusa.org.

Why we're still recommending fencing + a crossing, not the cheaper surface

To be direct about the tradeoff the table above shows: at low coverage, every surface material is cheaper than fencing. We're not hiding that, and we think a low-cost pilot is worth trying. But cost isn't the only variable, and on the two that matter most for a DOT decision, fencing + a crossing wins outright:

• Proof. Fencing + crossing structures have decades of published before/after collision data behind the 80–100% reduction figure. The red-surface treatment is new (2026), and we could not find independent collision data confirming it works at all — only the design rationale. A closely related idea, reflective roadside markers meant to startle deer at night (Swareflex reflectors, widely used in Europe), looked just as promising on paper and then failed controlled studies: one Wyoming study found deer behavior unchanged, and actually more deer killed with the reflectors active than covered.
• What it actually fixes. To be precise: animals can still cross a road with a surface treatment on it — nothing physically stops them. But they cross exactly the same way as today, at grade, in the same lanes as moving traffic, with no separation from vehicles. A surface treatment might change driver behavior a little at that spot (unproven, per the evidence gap above), but it doesn't remove that exposure, and it doesn't touch the barrier effect Marcel Huijser described to us: some species avoid a busy, disturbed road and won't attempt the crossing at all, regardless of pavement color, which is what actually isolates a population from habitat on the other side. A crossing structure, away from traffic entirely, is the one option that removes both problems at once.

So our actual recommendation is: fencing + crossing is the only option here backed by real evidence and the only one that solves the whole problem, and this corridor's scheduled rebuild is the cheapest window there will ever be to add it. The surface treatments are worth a cheap, honest pilot at a single bad spot in parallel — not a substitute for the crossing study, but not a bad idea either.

Other approaches, from elsewhere in the world

• Canopy/rope bridges (Brazilian Amazon, Australia, Peru) — rope lines strung across the road canopy for tree-dwelling species. Astonishingly cheap (a few hundred dollars each) and effective where they fit: one Amazon highway logged 15,000 crossings and zero roadkill over 15 months. Doesn't apply to us directly — it's built for monkeys and gliders, and Iowa's problem is almost entirely ground-dwelling white-tailed deer — but it's the cheapest wildlife-crossing intervention in the world where the species fits.
• AI/sensor-triggered roadside warning signs (Florida's RADS, Nevada's RADIS, radar-based systems) — detect an animal approaching the road in real time and light up a warning sign only when something is actually there, rather than a static seasonal sign. More targeted than what we're doing with the app, but it's infrastructure the DOT installs and maintains, not something a map can substitute for.
• Wildlife warning reflectors — worth naming so it doesn't come up as a cheaper suggestion later: well-studied in Europe and the U.S., and the evidence is bad. Multiple controlled studies found no change in deer behavior, and a Wyoming study found more deer killed with them active than covered.

What we're asking for

• A review of the US 61 / Louisa County corridor (project 11-58-061-010) for wildlife fencing and a crossing structure as part of the 2027–2031 rebuild already scheduled there — the most economical time to add it.
• If a full fence and crossing isn't feasible everywhere, consideration of the lower-cost surface-treatment options above (including the cheaper aggregates) as an interim step, understood as a driver-behavior measure, not a substitute for a crossing.
• A point of contact so we can share the underlying crash data and answer questions about how it was compiled.
• The opportunity to present this in person, if useful — we're a student team and can work around your schedule.

We're also reaching out to Keith Knapp at Iowa State's InTrans, who directs the Deer-Vehicle Crash Information Research Center that Iowa DOT already helps fund — happy to go through that introduction first if it's an easier starting point than a cold email.

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