When a public road or highway or portion thereof laid out over one or more tracts of land by the surveyors of the highways shall have been unworked and unused for a period of not less than twenty years, it shall be vacated if and when the owner or owners of the lands over which it was laid out shall have filed in the office of the clerk of the county wherein the road lies, their assent in writing to such vacation, proven or acknowledged before an officer authorized to take the proof and acknowledgment of deeds and indorsed by him with his certificate.