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Interstate 6 has never existed.
I-106 is an early California proposed number inconsistent with the interstate highway numbering convention.
I-106 (preliminary numbering) California
On April 1, 1958, California submitted to AASHO a numbering plan for its proposed interstates, including 3-digit spurs numbered in the low 100s and not associated with their parent interstates. Interstate 106 was the proposed number for what became the old I-105 and I-110, where the Santa Ana and San Bernardino freeways meet. 
This number was not compliant with AASHO's numbering convention and was rejected.
On August 7, 1958, California had bought in to the system and submitted I-105 and I-110. AASHO wanted to combine them into a continuous I-110, but the state replied that ramp configurations made it impossible for motorists to follow the entire route. Hence the two spur designations. 
These numbers were approved, along with the rest of California's original interstate highway system, on Nov. 10, 1958.