Section 7B. Wherever in this chapter the department is authorized to take land by eminent domain under chapter seventy-nine, in connection with the laying out, widening or relocating of a public way, the department shall be authorized to take, or institute proceedings for the taking of, an easement in land adjoining the location of the public way consisting of the right to have the land of the location protected by having the surface of the adjoining land slope from the boundary of the location.