Oregon Revised Statutes Highways, Roads, Bridges and Ferries § 371.410

(1) A road assessment district may be formed in any county having a population of 19,000 and not more than 25,000, as shown by the decennial federal census, and shall consist of an area of more than 20,000 acres or an assessed valuation of taxable property of not less than $1 million, according to the last county assessment roll. A road assessment district may be formed to provide for the improvement, repair or reconstruction of the public roads within such area of land.

(2) A road assessment district formed under subsection (1) of this section shall continue in existence under the provisions of ORS 371.405 to 371.535 notwithstanding a growth in population exceeding 25,000, as shown by a subsequent decennial federal census, in the county where the road district was formed.