These icons () show footnotes as tooltips... for some browsers.
72.15 miles ; the 405 (San Diego Fwy) serves the San Fernando Valley, the beaches, and Orange County.
I-405 was approved as a chargeable interstate on Sept. 15, 1955.   The first section of I-405 opened in 1957, signed as CA 7. The part west of I-605 was done before 1965; the newest section, near the southern junction with I-5, opened in 1969.  
When the number for this route was being decided (in the 1950s), California suggested "Interstate 9." 
30.30 miles ; goes east of Lake Washington to serve suburbs of Seattle.
The idea for a bypass road is at least 60 years old. When the US 10 freeway (now I-90) from Seattle to Issaquah was built in 1940, designers put in a two-lane overpass where I-405 would eventually be built. The overpass sat unused for 14 years (this is not, however, anywhere close to a record.)
The portion of I-405 from NE 160th St. to what is now Sunset Blvd opened in stages from 1954 to 1957. It was originally called "Secondary State Hwy 2A" (SSH-2A) until 1964, when it became State Route 405. It was to have been posted US 99E, but by 1964 the 405 signs were replacing WA 1 signs. The section from I-90 to Kirkland was 4 lanes, as was the section between 112th and Sunset Blvd in the south end; the remainder was 2 lanes, undivided. 
The southern end was upgraded by 1965, and the north end was connected to I-5 in 1968. The section in between 160th St. and Kirkland was widened to six lanes around 1971. 
Severe congestion on Interstate 405 has triggered a dust-off of a 1960s proposal for an outer bypass called Route 605. The fate of this route is far from certain.
See also: I-405 (Paul Henry)