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I-405 California (link)72.15 miles [1 I-405 was approved as a chargeable interstate on Sept. 15, 1955. [3 When the number for this route was being decided (in the 1950s), California
suggested "Interstate 9." [3 See also:
I-405 Oregon (link)3.53 miles [1 See also:
I-405 Washington (link)30.30 miles [6 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.
[6 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.
[10 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) Sources
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