There are 15 US Post Offices in Washington County serving a total of 318,433 residents equating to 21,229 residents per post office. It's estimated that approximately 508,255 packages are handled across the 15 county post offices annually. Washington County is one of 36 counties in the U.S. state of Oregon. As of the 2010 census, the population was 529,710, making it the second most populous county in the state. The county seat and largest city is Hillsboro. Washington County is included in the Portland-Vancouver-Hillsboro, OR-WA Metropolitan Statistical Area. Cities in Washington County include Hillsboro, Beaverton, Tigard, and Forest Grove, the oldest city in the county. Originally named Twality when created in 1843, the territorial legislature renamed it for the first president of the United States, George Washington, in 1849. The original boundaries included the entire northwest corner of Oregon before sections became new counties. The Tualatin River and its drainage basin are almost entirely within the county, with the county nearly coterminous with the Tualatin Valley. It is bordered on the west and north by the Northern Oregon Coast Range, on the south by the Chehalem Mountains, and on the north and east by the Tualatin Mountains (or West Hills). Major roads in the county include small sections of Interstate 5 and Interstate 205, the Sunset Highway, Oregon Route 217, Oregon Route 47, Oregon Route 10, Oregon Route 6, and Oregon Route 8. Public transportation is primarily operated by TriMet and includes buses, the Westside Express Service commuter rail, and MAX Light Rail. Other transportation includes air travel at the Hillsboro Airport, several private airfields and heliports, and heavy rail cargo on several rail lines."