I-80 and its auxiliary freeways in California are anomalies. They had only so many numbers they could use (180, 280, ... 980), so that's why you have the break in convention. But otherwise, it has nothing to do with whether the freeway connects to the CBD.
From the Wikipedia page:
Quote:
Spur routes deviate from their parent and do not return; these are given an odd first digit. Circumferential and radial loop routes return to Interstate Highways, and are given an even first digit.
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Notice the wording "parent route." Note that I-215 begins at I-80 and ends at I-15, which means it never technically returns to its parent route. However, it returns to another Interstate freeway, which is really all that matters in receiving that even digit.
If there were an Interstate freeway spur that were built from, say, Bluffdale to Riverton, and it ran from I-15 to a surface street, or even SR-85 (an eventual freeway, but not an
Interstate freeway), it would still receive an odd-numbered first digit.
This diagram sums it up nicely: