The board may acquire lands for a right of way for a proposed new street which shall have been designated as a primary or secondary arterial highway as hereinbefore provided or may acquire lands to widen or change the route of an existing street which has been designated as a primary or secondary arterial highway as hereinbefore provided and pay the costs thereof or its share of the costs thereof in like manner as it is authorized by law to acquire lands for rights of way for county roads.