US 101 is designated as the Oregon Coast Highway No. 9 ("see Oregon highways and routes"), as it serves the Oregon Coast region. Much of the highway runs between the Pacific Ocean and the Oregon Coast Range, thus US 101 is frequently mountainous in character. For most of its length it is a two-lane undivided highway. Many parts of the highway are subject to closure due to landslides caused by excessive rainfall, and in many parts of the coast, US 101 is the only viable route connecting certain coastal communities. Thus, in many cases when landslides block US 101, the detour requires traveling inland over the Coast Range to alternative north–south routes in the Willamette Valley and then back west over the Coast Range again.