They build overpasses for prey animals, which prefer the open air and light.
They build underpasses for predators, which are more stealthy.
A company just finished a seventeen mile bypass around Scioto County, Ohio. No animal considerations that I can see. I wonder how much animal/vehicle conflict there has been. In the years of construction I imagined the wildlife learning to use the roadway as a transit path, drinking from the waterholes, eating the newly planted grass, traveling to and from where they've always traveled, then, suddenly, one day it was opened to 42,000 pound trucks hurtling along at 70 miles per hour, and two-ton cars, heavier by moving at 70 mph. I have not heard of conflicts but did see a car stopped and saw a bloodied deer get up and run away, on one underpass of another highway under the new one, and saw a deer near the exit of that same intersection of another occasion. Deer, bear, opossum, skunks, raccoons, lynx, could all be potential conflicts for the drivers.https://www.youtube.com/watch?v=RFCrJleggrI