The board of supervisors of any county may, by a resolution adopted by a four-fifths vote of its members, determine that any of the following activities are of general county interest and that county aid shall be extended therefor:

1. (a) The laying out, opening, construction, improvement, maintenance, repair, or altering of all or any part of any street within a city or extending along or across the boundary of a city.
2. (b) The establishing, modifying or changing the grade of any such street.
3. (c) The separation of the grades of any two or more such streets which intersect.
4. (d) The separation of the grade of any such street from the grade of any steam, electric or street railroad crossing such street.
5. (e) The construction of the necessary pavements, curbs, culverts, bridges, tunnels, subways, viaducts, drainage facilities or structures incident to or a part of such street.
6. (f) The acquisition of any real property or interest therein, rights of way or other property necessary for any of the purposes mentioned in this section.