Mont. Code Ann. § 7-12-4131

All streets, alleys, places, or courts in the municipalities of this state, now open or dedicated or which may hereafter be opened or dedicated to public use, are open public streets, alleys, places, or courts for the purposes of this part. The city council of each municipality may establish and change the grades of the streets, alleys, places, or courts and fix the width thereof and may acquire private property for right-of-way and order to be done any of the work mentioned in this part under the proceedings described in this part.