The 3-digit Interstates are "loop" and "branch" routes that encircle big cities (like I-465 around Indianapolis), or bypass them to interconnect inter-city freeways (like the x80 series in the SF Bay Area).
The numbers are formed by prefixing a digit to the number of the main Interstate route they connect to.
Yep, and that leading digit indicates a loop if it's an even number, or a spur if it's an odd number. The N/S/E/W designation is supposed
to follow that of the "parent" interstate.
There was a bit of discussion when I565 in north Alabama opened. I65 is a N-S route, so by definition 565 is a N-S route as well. The problem is, 565 is about 20 miles long but only covers about 2 miles north to south. Very confusing, even for locals, especially for visitors. The highway department finally relented and relabeled it E-W.