For most of us, roadkill is an occasional, gruesome nuisance.

But around the country, it adds up. The 250 million cars and trucks continuously traveling America's roads kill unknown millions of animals a year — and there are some good reasons why we shouldn't ignore this entirely.

To the average person, roadkill is one of the few times we regularly see wild animals. To ecologists, roadkill data is a valid indicator of the diversity of animals in an area — and a sign of how much our roads impact them.

1) Roadkill has been a fact of life for centuries

Horse-drawn carriages, trains, and other vehicles have been killing animals for centuries. But as the automobile spread during the first few decades of the 20th century, roadkill rates spiked — and scientists started to take notice.

"This is a relatively new source of fatality; and if one were to estimate the entire mileage of such roads in the state, the mortality must mount into the hundreds and perhaps thousands every 24 hours," California naturalist Joseph Grinnell noted as early as 1920.

In the 1930s, ecologists began collecting hard data on roadkill (which was sometimes called "flat meat," a reference to the fact that it was more frequently eaten). A landmark 1951 study identified one of the biggest factors affecting its frequency: improved roads — which allow cars to travel faster — typically lead to more roadkill.

2) Everywhere there are cars, there is a ton of roadkill

No one really knows how often animals are killed by cars in the US. But one thing's clear: it happens a lot.

There are about 253,000 reported animal-vehicle accidents per year (that is, accidents that are substantial enough to cause damage to the car). Last year, State Farm estimated that about 1.2 million deer were killed by cars in total.

When you factor in small animals, the number climbs dramatically. No researchers have done a thorough nationwide count, but very rough estimates are that around 365 million vertebrates are killed per year.

3) Scientists use roadkill to track invasive species

One positive to all this roadkill? "I sometimes think of roads as a continuous wildlife sampling device," says Fraser Shilling, a professor at UC Davis who operates the California Roadkill Observation System, a database of 29,527 instances of roadkill across the state since 2009. Because the animals that die on roads are fairly representative of the animals present in an area, this database can be used to learn about California's vertebrates as a whole.

And because invasive species are just as likely to get hit by cars as any other, Shilling has used the database to map the spread of the Eastern gray squirrel and Eastern fox squirrel in California. "They're both invading different parts of the Western gray squirrel's habitat, and we can see that in the roadkill data," he says.

Ecologists have also used roadkill data to map the spread of the red fox in Tasmania and Burmese pythons in the Everglades.

4) Some people intentionally drive over animals

A few studies have revealed an uncomfortable truth about human behavior: some of us, it seems, intentionally swerve to hit animals.

In one Canadian study, scientists put a fake snake, a fake turtle, or a piece of garbage (say, a Styrofoam cup) on a highway, and found the fake animals got hit a disproportionate amount of the time — leading the scientists to calculate that 2.7 percent of drivers were intentionally hitting them. This same trend has also been seen in Brazil and Australia.

5) Road salt and roadkill can lure animals to the road — and cause more roadkill

Salting the roads during winter leads to all sorts of negative environmental effects. And in some places, researchers have found, the salt accumulates in the rumble strips paved in the shoulders of highways, attracting animals like moose who want to lick it — and causing them to become roadkill.

Meanwhile, in some cases, it's roadkill itself that attracts animals to the road. In one South African study, researchers found that spilled grain along one stretch of highway frequently lured mice. After they were hit by cars, their carcasses attracted scavenger birds, some of which became roadkill, too.

6) Fences alone can't prevent roadkill

Starting in the 1950s, road engineers devised a solution for roadkill: keeping animals off roads. Building fences, the thinking went, would exclude animals from road surfaces, allowing cars to travel quickly to their destinations without interruption.

Except for one thing: these fences can cut down on roadkill, but researchers have found that it spikes near the ends of the fence. When searching for food or migrating, after all, animals often need to cross roads to survive, and they simply go around the fences.

In many places, engineers have since taken up a new philosophy: trying to accommodate animals with underpasses and other sorts of planned crossings. Most have been built in Western states, and have often been designed to save endangered species in particular.

Correction: This article previously included a miscalculated statistic about the rate of roadkill production in the US.