Rock chips can be greatly reduced by not following other vehicles too close, and allowing a "cushion" of space so that flying rocks will hit the ground in front of you before reaching your vehicle. Most hard-shell lens covers tend to distort/deminish light output of the headlights; thereby, degrading night-time visibility.
There is also a protective "film" available that can be custom-cut and applied to headlights/foglights; along with other cool stuff for motor vehicle enthusiasts, check out:
www.griotsgarage.com