Regarding three-digit interstates
: generally if the number starts with an ODD number
(105, 305, 505, etc) the route
is a spur
off of the "parent" interstate
. if the number starts with and EVEN number
(205, 405, 605, etc) the route is a loop
off of the parent road, which will return at some point.
There are exceptions, notably in the SF bay area, where a lack of parent interstates (only Interstate 80) combined with a plethora of interstates, means that the x80 routes don't always follow these rules. Indeed, several don't even intersect I-80!
Regarding route numbers
in general: there are exceptions to the rule. The most glaring is Interstate 99
, so numbered because the local congressman
thought it would be nifty.
Another major "error" is Interstate 238, again in the bay area. As described above, they had a hard time coming up with numbers for the local freeways in the area, so they simply turned what had been CA 238 into I-238. This highway is nowhere near where an Interstate 38 would be, if there were a I-38 in the first place!
There are other exceptions, but these are the most exceptional.