FLORENCE — If you’ve never seen a meadow vole cross Highway 93 near Bass Creek, that’s because you’ve never seen the ramp it gets to use.

Hidden in the cattails at the base of the barrow pits along this high-speed, four-lane roadway, a network of two dozen culverts have been modified so little critters can make their east-west passage against the north-south traffic. Motorists can’t miss the huge “Animals’ Bridge” overpass north of Evaro Hill. Mice, rabbits, raccoons and weasels show equal ease finding their way through their own underground shelving.

“On the first day we installed one, I set a tracking plate to see when animals would start using it,” said Kerry Foresman, a retired University of Montana wildlife biologist and designer of the patented “Critter Crossing Technology” shelves. “The next morning we had hundreds of tracks in a matter of eight hours. Voles were cruising back and forth. They’re comfortable in a dark tube like this.”

The soggy fields between Lolo and Hamilton put lots of erosion pressure on Highway 93, especially in spring when the Bitterroot Mountains shed their snow in runoff surges. Culverts divert that water under the roadbed instead of over it. While some of the bigger streams like Bass Creek have underpass bridges, many smaller tributaries flow through steel or concrete tubes four to 12 feet in diameter.

During dry months, many mammals willingly walk through the culverts. But they can’t handle the swifter spring flows, or the ice-lined winter months. Tiny mammals, like field mice and voles, usually won’t use them at all because they feel too exposed to predators.

When the Montana Department of Transportation was rebuilding Highway 93 through the Bitterroot Valley, Foresman started getting calls from concerned residents and road engineers about how the project would affect wildlife. Lots of animals got killed trying to cross two lanes. Doubling that, and adding a center turn lane in some areas, would amplify the carnage.

“We can build bridges for elk and deer, but what about coyotes and foxes?” Foresman said of the challenge. Federal transportation researchers estimate that more than a million animals a year die on the nation’s roadways. Several thousand humans also get injured attempting to avoid those collisions, and hundreds die in the resulting wrecks. Highways also slash the genetic diversity of species, including meadow creatures like the Preble’s jumping mouse in Colorado, which is on the federal Endangered Species List. So the problem had real urgency for a solution.

Foresman’s idea was to hang a shelf along the inside of the culvert, above all but the highest water level. He got in touch with Cory Clausen at Missoula’s True North Steel for material advice.

“We came up with a structural steel fabric that was modular and removable,” Clausen said. “Kerry had the ideas and we support him with the appropriate products.”

But several challenges remained. Transportation hydrologists — the engineers who design road drainage systems — worried the shelves might plug their culverts. Clausen came up with a mesh panel that water can flow through. But would animals willingly walk on mesh?

Some will and some won’t. Felines, whether bobcats or house cats, slink right across the grating. But canines such as coyotes and foxes balk. Sometimes they’ll tightrope their way along the solid rims of the grate. Porcupines, raccoons and marmots march on in.

Little rodents have a different issue. Thousands of them chase about in burrows and gaps under the meadow grass, never exposing themselves to a sky full of owls and hawks. No way were they going to walk above the grass on a steel ramp like hors d’oeuvres on a platter.

So Foresman went to a hardware store for a length of drainpipe to see if the rodents would use a more discrete and confined passage. They did. So he conferred with Clausen some more. Clausen added a metal tube to the side of mesh grates, which gave the little critters their own private expressway across the road.

While humans typically enter dark, confined places with no end in sight only when holding a horror-movie ticket, it was the rodent tunnels that gave Foresman the most immediate proof of concept on his shelving design. Theirs were the footprints on his tracking plates that appeared within hours of installation. It took much longer for remote-trigger cameras to capture images of larger mammals using the shelves.

“We were here every few days for three years,” Foresman said of the testing and installation process. “All those clumps of trees along the highway are where we have shelves. That’s where the water flows through.”

The success of the Bitterroot shelves has attracted attention elsewhere. New York transportation officials teamed up with the Nature Conservancy this summer to install a culvert shelf along State Route 12 in the Black River Valley.

“They have the same problems we have here,” Foresman said, “just magnified by more people.”