It is hereby declared to be the policy of the Commonwealth that all crossings of one railroad by another, or a public highway by a railroad, or a railroad by a public highway, shall, wherever reasonably practicable, pass above or below the existing facility. And every railroad hereafter constructed across another railroad or across a public highway, and every public highway hereafter constructed across a railroad, shall, wherever it is reasonably practicable, and does not involve an unreasonable expense, all the circumstances of the case considered, pass above or beneath the existing structure at a sufficient elevation or depression, as the case may be, with easy grades, so as to admit of safe speedy travel over each. If constructing a crossing either above or below the existing structure is not practical and involves an unreasonable expense, the responsible governing body constructing a new public crossing at grade, in accordance with the laws of the Commonwealth of Virginia, shall take precautions to provide for the safe movement of traffic. It is the policy of the Commonwealth to limit the number of new public at grade crossings and to eliminate unnecessary crossings.