The great irony of roadkill is this: Its most conspicuous victims tend to be those least in need of saving. Simple probability dictates that you’re more likely to collide with a common animal—​a squirrel, a raccoon, a white-​tailed deer—​than a scarce one. The roadside dead tend to be culled from the ranks of the urban, the resilient, the ubiquitous. But roadkill is also a culprit in our planet’s current mass die-​off. Every year American cars hit more than 1 million large animals, such as deer, elk, and moose, and as many as 340 million birds; across the continent, roadkill may claim the lives of billions of pollinating insects. The ranks of the victims include many endangered species: One 2008 congressional report found that traffic existentially threatens at least 21 critters in the U.S., including the Houston toad and the Hawaiian goose. If the last-ever California tiger salamander shuffles off this mortal coil, the odds are decent that it will happen on rain-​slick blacktop one damp spring night. The poster groups for roadkill’s hidden toll are …