Feasible paths for animals to cross roads/highways

There is roadkill everywhere. Im constantly heartbroken seeing the bodies strewn across roadways throughout America just because people both dont care to stop and because they have no safe option?(the animal's) to venture across to the other side. I propose making either little bridges that span across roads and highways or the alternative coule be underneath them. Creating underway's with which the animals could use to get safely to the other side