605 ILCS 5/9-118
Any association, society, person or persons may, upon obtaining a permit from the highway authorities having jurisdiction over the particular highway, and the consent in writing of the owners of adjacent property, plant or set out trees, shrubs, plants or flowers in or upon the right-of-way of any highway within this State, provided that no such tree, shrub, or flower shall be permitted to obstruct the vision of persons traveling upon or across such highways. Consent in writing of the owners of the adjacent property shall not be mandatory, as the highway authority shall determine, for plantings in a median or in those portions of a highway where direct access from the adjacent property is not provided. The highway authority having jurisdiction may place suitable signs giving notice of the association, society, person, or persons doing the planting.
605 ILCS 5/9-118