[amyjess]

When the Interstate Highway System was first created, AASHTO had a rule that no Interstate and US Route with the same numbers could coexist in a state. Since with both numbering schemes, 50 and 60 would fall in roughly the same part of the country, I-50 and I-60 had to be skipped so they wouldn't share any states with Route 50 and Route 60.

AASHTO has since relaxed that rule, and there are now a few exceptions to it, but it's too late to shoehorn in two major interstates.

[jrapdx3]

AFAIK, the old US Highways were numbered opposite to the Interstate system, at least for round-numbered routes. US-10, 20, 30, etc., more or less proceeded north to south, Interstates are increasing south to north. The middle numbers like 50 would possibly overlap in both systems so designations like I-50 weren't used.

[amyjess]

Indeed they are. Route 1 mostly runs along the east cost (key word is "mostly") often next to I-95, and Route 99 used to exist right next to where I-5 is now (it's since been turned into a gaggle of state highways in CA, OR, and WA), while Route 2 runs in the north and Route 90 largely runs next to (or is co-signed with) I-10.

[jrapdx3]

Yup, in fact there are 99W and 99E more or less straddling I-5 and running in generally north-south alignment in the Portland metro area. In recent years I-5 has become increasingly congested, not just in "rush hour". Consequently 99E/W are also heavily used as commuters try to escape traffic. Hard to solve the problems created by our success.