SR 129 is the southeasternmost highway in Washington state and is maintained by the Washington State Department of Transportation (WSDOT). Most of the rural highway is two lanes wide, with no shoulder, and has a posted speed limit of outside of towns and cities. WSDOT conducts an annual survey on state highways to measure traffic volume in terms of average annual daily traffic. Average traffic volumes on SR 129 in 2016 ranged from a minimum of 180 vehicles near the Oregon state line to a maximum of 9,800 in downtown Clarkston.