The Snake Road

Discovery News reports that snakes in the southwestern United States have a major problem: They refuse to cross roads and are becoming severely inbred as a result of this.
Rattlesnakes are a common species in the southwest, being a brown snake that eats rats, mice, and opossums, and having a collection of dried skin at the end of their tail that makes a rattling noise when shaken, hence the name. They shake their tails when they feel mildly threatened as a warning, and if that doesn't work, they have a poisonous bite. Hikers in the southwest are instructed not to provoke them, as they don't bite arbitrarily.
When more severely threatened, by loud noise or severe vibration, they lay perfectly still and hope that their camouflage will leave them undetected. This works great when hiding in the grass and the threat is a screaming hawk. This works less great on an asphalt road when the threat is a large car. The car's drivers typically never even sees the snake, and the snake's life ends with a loud thump that the driver can't tell apart from hitting a rock.
As a result, the surviving snakes have come to the conclusion that road equals squishy death, and refuse to cross them. The result of this and expanding roads have meant that snakes live in the little pockets between them and do not cross the road for any reason. Snakes will not mate with a snake a mere 500 meters away due to a road between them, and have gotten very incestuous as a result.
I know most people will just shrug. "Who cares about snakes?" To which I respond, "You like eating, don't you?" Snakes are the main rodent-control of the southwest, eating the vermin that would otherwise threaten crops. If they become too inept or sickly to eat these mice, then local farmers will need to invest in an expensive series of rodent-controls.
Discovery's biology department proposes that roads be built with periodic bridges, as snakes are perfectly willing to travel under a bridge.