u.s. route 101, or u.s. highway 101, (us 101) is a north–south united states numbered highway that goes along the west coast of the united states. it goes through the states of california, oregon, and washington. it goes alongside, and often joins onto, california state route 1 when going through california. unlike most other 3 digit routes in the system which are treated as spurs of their two digit parent routes, us 101 is treated as a two digit route with a first digit of "10". the reason for this was that if the west coast highway was numbered us 91, there would be no room to establish us 93, us 95, us 97, and us 99, all of which must be west of us 91. so, they decided to call the west coast highway us 101 instead. the '10' could have been underlined so that people think that this route is a two digit route rather than a spur route of us 1, the us highway on the east coast.