Wildlife Passage Design Calculator

Recommended Width and Height come entirely from Target Species Size -- an animal needs a minimum opening it's willing to use regardless of how wide the road above or below it is. Road Width instead affects how long the passage structure must be to span the road, which shows up in Openness Ratio (cross-section area divided by length), and -- for culvert, overpass, and bridge adaptation passages -- in cost, since those three are priced by length. The default Underpass passage type is the exception: it's priced by opening area, so Road Width doesn't move its cost at all, only its Openness Ratio. A wider road increases the length the structure must span without changing the cross-section area, so a longer passage for the same opening size feels narrower and more tunnel-like to wildlife -- which is why wider roads produce lower openness ratios.

No -- Traffic Volume has no effect on Recommended Width, which is set entirely by Target Species Size. Traffic Volume instead feeds into the collision-economics side of the model: how many vehicle-wildlife collisions are estimated per year and how much the crossing might save by reducing them.

It's a widely used road-ecology metric -- cross-section area divided by passage length -- that correlates with how willing wildlife are to use an underpass. This calculator flags a ratio above 0.6 as generally adequate for large mammals and above 1.5 as excellent, while lower ratios suggest a passage that may feel too enclosed for target species to use reliably.

No -- this is a planning-level estimate using general dimension and cost guidelines from road-ecology literature. A real wildlife crossing project needs site-specific structural engineering, environmental permitting, and often species-specific behavioral studies (camera trap monitoring, movement corridor mapping) that this calculator does not attempt to model.

This calculator estimates annual wildlife-vehicle collisions from Average Daily Traffic -- roughly one collision per 5,000 vehicles/day, with a floor of one per year -- then multiplies that by a typical per-collision cost that scales with Target Species Size (about $500 for small animals up to $15,000 for very large ones like elk or moose) and by the Expected Collision Reduction the crossing is projected to deliver. Neither the collision-rate assumption nor the per-collision cost table is a site-specific figure; treat Payback Period as a ballpark planning figure, not a guaranteed return on the crossing's cost.